You can either get to like it as it is, which costs roughly nothing, or...
There were many performance upgrades available for the 400F in the 70's, and there are still quite a few available.
Basically you have camshafts, intake/exhaust, and displacement. (You can do some changes to the transmission but outside of racing use this is a bit pointless)
Each of the three adds a bit of power if chosen carefully, but for a real change you have to do all three.
Yoshimura R&D and Ontario Motor Tech both had go-fast parts for the bike. Yoshimura turned to Suzuki and dumped any 400 stuff about 20 years ago. Kaz Yashima and OMT are also long gone.
Kevin sells a bunch of stuff that MCAgain in Japan has for the 400, you can check their website (mcagain.com I think) for some amazing pictures but it's in Japanese.
MCAgain somehow had Yoshimura 466 piston/ring kits and still have head gaskets for them: Yoshimura made a 458 kit with the 466 essentially as an overbore for when you wore out a 458 kit; people just bought the 466 kit though, the 458 never sold so well. Kevin has a Wiseco 466 kit now, I guess the Yosh ones ran out or the price was ridiculous. (ummm... the wiseco ikit s $760.00!!! Ouch!)
This overbore makes a big difference, especially if you add a mild cam and some easier breathing.
Yoshimura also had three stages of hotter cams. OMT had two or three. Not available now, of course. Kevin has been trying to provide a Yosh stage one cam, but I don't see it listed. Webcam and Megacycle both have interesting cams available as exchange (they want a usable cam to modify) and either brand is good in my opinion. A "street" cam is best for normal use, you lose a lot of rideability when going to a full race cam. The cam makes some difference with a stock engine. Along with the 466 kit it is a big improvement.
Breathing is important. The stock header looks sexy but the unequal length tubes compromise performance. There aren't too many good choices for pipes nowadays, Kevin shows the cheapest MCAgain one and can get you the pricier ones, they are damned loud but work well. Using a Supertrapp or other quieter muffler would work, but it would be a shame to cut the thing. If you can find a used Kerker it's quite good and will fit a Supertrapp (the Kerker mufflers rusted out really fast). MAC has a pipe that works fine as well, quality is rough but it isn't ridiculously expensive.
Porting is always nice, just a bit of matching the outside openings to the carb insulator holes and header diameters plus some smoothing of the rough casting helps. More aggressive porting should be left to experts: what you think will improve flow may reduce it drastically. The airflow curving as it hits the valve guide complicates porting a lot, the issues are well known to porting gurus who work on these designs but guys who work with modern straight path engines don't understand
The carbs are a limiting factor for power at high RPMs especially with an overbore. Rick Denoon will bore your stock carbs out to roughly double the flow capacity, or you can try KeiHin smoothbore CR specials. The CRs are a bugger to fit and give just adequate performance at lower RPMs... probably best reserved for racing. At WOT throttle near redline the difference between stock carbs and the CRs is breathtaking. At least the newer ones have a choke system, the 70's chokeless ones were very very tricky to start on a cold engine. I've heard good stuuf about the Denoon bore-out approach, low end is still much like stock while the top end is radically improved. Doing the boring requires the carb bodies to be removed from the quad rack and completely stripped.
The stock air filter is resonably good but the airbox itself sucks. The tiny intake snorkel is restrictive at high flow. using "pods" like K&N frees the restriction but they have problems too: you will have carb tuning "issues" getting the low end and the transition to mid throttle running properly, the carb is tuned for the flow and vacuum from the stock airbox and "velocity stack" runners. I personally like the K&N filter pods but many folks here hate them.
You have changed the gearing, good start. The stock engine can't pull redline in top gear so increasing the final drive ratio is the cheapest way to get a hotter feel from the bike. With a tricked out motor you should manage around 120mph, gearing so redline is around there will be the best performance but you'll have a pretty high rpm for cruising at 60-80mph.
Pricing out the full workover is scary. You can buy a decent used bike for the money that will be quicker and faster than any 400f can be.
