POOR PERFORMANCE

I read that you have original exhausts but are they louder than normal? PO cut the baffles or they've rotted. The reason I ask is I had my 1st Zeph11 as a kid and it would hit 142mph in standard trim. Then the exhaust balance pipe rotted and I got some drag pipes. Went like the the bloody clappers and wheelied!! but wouldn't go above 125. Then got a set of Harris pipes, Better but with the race baffle would only hit 135. Too much free flowing air without rejetting. The carbs did have a dynojet kit but obvioulsy not enough fuel top end

It would only take one tooth extra on the front OR 3 teeth less on the rear from stock to effectively make your 4th gear equivalent to a stock 5th gear ratio.
Which would mean your 5th gear  is now equivalent to a 6th gear ratio which your engine output was never designed to run. 
If your bike has  one tooth bigger front and three teeth smaller on the rear I wouldn't be surprised it struggles in top. 

Gonna be a few days till its warm enough to ride to the shop to do a count. Ill report back soon as i know.