Put gen4 and gen5 gear ratios into Excel table and multiply gen4 values by 43/40.
When geared 3 teeth differenty at rear, you see following:
At 2nd and 5th both have same speed.
Gen5 has taller 6th (good) and taller 1st (bad).
If no motivation messing with kit gears, then an interesting idea would be using gen4's 5th and 6th gears in Gen5 box, that would make bigger jump going from 4th into 5th.
Gen4 race: high spec.
Suzuki GSX1300R Hayabusa 2013: stock.
Kawasaki KX450F 2016: love it.