Thanks. I'm probably not a good reference because this is the only CB750 I've ever ridden and I can't make a good baseline comparison. I can only tell you about my experience.
I will say the Cognito Moto airbox is a beautiful piece. I contacted them and got some dimensions because I knew I would have clearance issues. I welded my key switch housing into the upper corner of the frame in front of the oil tank. I knew I wasn't going to run a stock airbox, but didn't do a full engine mock up installed in the frame prior to all the paint work. What I got from CM was a 750 airbox with a shorter 550 element and screen. I still had clearance issues so I had a buddy mill down the mounting/flange plate, then I had both plates powder coated gloss black. The matte black anodized finish didn't match the bike.
As far as performance goes, I had bought a jet kit that didn't work out so well and I had a mismatched #2 carb body. Once I got the right carb body I installed 110 jets and the bike was running fine. I bumped the jets up to 115s out of curiosity, but was getting too much gas. I reinstalled the 110 jets and I'm going to leave it at that. I don't know about any dead spots. The bike has quick throttle response and now that I've got a 48 tooth rear sprocket it pulls better.