I have not checked compression on this engine. Something I can do, I guess, but it runs like stink! Really strong.
Removing the filter is an easy test. I also plan on trying to back the air screws way out....four turns or so....and then blip the throttle. If it returns to idle and doesn't hang, I'll know the slow jet is really rich. Correct?
Will check for air leaks and confirm jet sizes this weekend.
That's backward: turning them out makes them richer. Turning them in shuts them off. 
We continue to disagree on this point, unfortunately. I'm not sure why, as both the physics and my exhaust gas analyzer say that turning the air bleed screws outward makes the mixture leaner. Further, if the air screws are hollow tipped and cross drilled, it is impossible to "shut them off" as the hollow tips and cross drilling become a bypass for air path feeding the pilot emulsion tubes (Air source is the pilot air jet orifice).
I've skimmed this thread, and have yet to learn what slide needle (Jet Needle or JN) settings were chosen for this set of carbs, and if the slide needles have the stock numbers printed upon them, or if the emulsion tube JN jet has been altered from stock.
087a carbs seem to have been a baby step toward the lean burn engines demanded by the EPA back in the day. The UNI filters on my 550's have made the entire operating range of throttle a tad leaner (after I learned not to over oil them). They all have stock exhaust and induction components, but are now running on the hated ethanol gas (California allows on choice in the fuel matter, thank you meddling and overbearing democrats). But anyway, this makes the machine run a bit leaner. My bikes tolerate this except for two issues:
1 - Throttle twist response from low RPM becomes more sensitive. I've always maintained that the idle mixture was set over rich to accommodate throttle twist, with out adding an accelerator pump. My metric is that the air screws are set to allow the throttle to be twisted up to 1/2 of total remaining throttle in any gear (yes, even fifth) with the engine providing reliable (if not brisk) acceleration from idle RPM. Twist it more and you can expect stumble. The throttle twist limitation is overcome at 4000 and above because the venturi is more dominant at supplying the low pressure in the carb throat by that air speed, which allows more reliable and steady pulling of fuel from the fuel jet pathways.
However, an eighth (or maybe 3/16) of a turn inward on the Air bleed screws (from stock setting) restored the throttle response on my bikes.
The second issue is a far more pronounced hesitation at 3000-ish RPM. (For my F models, anyway. My 550K is in storage, but I never experienced any hesitation at any RPM with both the stock exhaust or a 4to2 system that I ran for 15 years. I'm not sure what the new fuel will do to this specific bike, as I've let it sit so long that it (sadly) now needs a restoration effort, as well as swing arm bushings).
If the MAC muffler is unaltered, the ones I've experienced had a similar back pressure profile to the stock 4 into 4 system used. It *should* be ok for the 087 carbs. However, I have no knowledge of MAC's quality control, or if there is any effort to maintain an equal pressure characteristic to the stock system, or even between their own production samples, or for that matter, eras of manufacture. Certainly the same guy that built them in the 70s is not the same guy that builds them these days. It may well be that the only specification of the system is that it mounts and looks the same on the OUTSIDE. Metal cutting flash, interior hole sizes, and other details about the muffler internals are hidden from the buyer's purview. And, when people buy solely on the basis of price or looks, QA efforts aren't rewarded and often eliminated.
For sure there is pressure differences between the 4to4 and the 4to1 which will be RPM modal, as 4to4s get little to no reflected pulses from other exhaust events, but a 4 into one gets reflected pulses from ALL the other cylinders. If tuned correctly, the 4 into 1 will provide a boost in the high power band of the engine, at the expense of fouling the mixture needed by the cylinder at some lower RPM operation. If the internal configuration of the 4 into 1 system is allowed to change in production, then both the high RPM boost parameter and the low speed fouling RPM can vary among samples. My take is that the MAC system pipe diameters and length are not specifically tuned for ANY RPM, they just borrowed the overall looks from the racers and made it a cheaply as possible. (The facade movement/demand has been present for decades.)
Having said all that about the exhaust, it may not be important, and it could simply be that the slide needle or throttle valve system is f'd up. I'd look there first, examine and measure the fuel metering orifices. Actually, if the bike were here, I'd just swap out the 087a with a set of 022a first, to isolate a carb issue from some other factor. I still have not been able to compare a virgin 087a set to what I know are virgin 022a settings and measurements. But, not everyone has a spare (and known) 022a set on a shelf in the garage.
JN S/B 272304
JN jet S/B 260mm
JN clip position is the 4th groove from top.
The throttle valve and JN system provide the dominant mixture control between 1/4 and 3/4 throttle position. Which is most of the "on street" operation. Poor fuel economy (less than 45mpg with non-ethanol gas) likely means that the throttle valve system is out of whack. (<--- technical term, right?)
Apart from the three basic JN items just listed for the throttle valve, there is also the air jet, the air jet passageway itself, and the emulsion tube air bleeds size, quantity and placement that can have lesser effects on run operation mixture.
There is another factor to mention, I suppose. (Honda did in their technical series on carbs of that era.) That of slide cutaway. If you thought the presence or shape of that feature incidental, you thought wrong. It has an effect on throttle transition between idle and about 1/4 throttle position. Seems it forms a secondary venturi during those throttle positions. Changing the ramp angle and/or size of the cutaway changes the vacuum level in that transition range. Of course, any change to the throat vacuum directly effect the volume of fuel drawn into the carb throat, making it seem like a jetting problem (which it is, indirectly).
Such are the details one must (or ought to) deal with when making changes to the stock configuration. (But, I can see your eyes rolling.)
Good luck!
