It depends on the power engine has.
You'll feel the difference, smooth slip or jerky movement by gearbox.
Drive chain OK, not too worn sprockets looking like a stormy sea and chain jump over the cogs...?
When my K6 had car oil of good brand in the 80's (Mobil 1)
It slipped when gearing to 5th at around 190kmh +WOT. I heard how it slipped, reduced throttle and wot again. It sounded like gearing to 6:th 
.
Same bike 2 years ago, slip on 2nd gear around 3000-3500 rpm. Much more torque with 1000cc.
That was the clutch lever with too little play. After that no chancing with 1/4 turn out. Always 1/2.
That bike is well worth to get this sorted.
In worst case open the cases with engine up-side down to fix eventual gear forks.
Before that: I should investigate if something with gearing mechanism inside the shifter cover can be involved.
To answer all your helpful comments:
The engine has good power, very responsive throttle, easy cold start, nice idle.
Jerky in second only, all other gears strong, no clutch slippage.
Drive chain new and sprockets not worn at all. Curiously though I set up the new chain for 1/2" of slack but just checking now it's about 1". Should a new chain stretch that much initially and would that be enough to jump sprockets? Did the jerking in second cause the chain to stretch that much?
Non energy saving oil, no friction modifiers, 10W40. What is your preferred brand and weight?
I believe I did 1/4 turn but will recheck and do 1/2 turn on the clutch lever.
Shift mechanism looks fine, see above pics.
I understand the ultimate fix is splitting the cases but I just don't have room/garage space to do that.
