[Racerxxxgsxr1000]

Long known, the K5-6 (my streetbike) has better midrange torque.
great for street and stunting wheelies etc....
Downfall is top end, noisy camchain tensioner assembly until warmed up, frame recall issues.

K7-8 (my trackbike) seems to have quieter camchain tensioning assembly, only slightly heavier mostly due to dual exhaust stock(no one keeps it stock) good midrange with better top end.
I believe the change in power curve was race inspired.
Too much low end torque coming out of corners (K5-6)made it wheelie prone and it ran out of big end power sooner.
The K7-8 will pick it up, but tends to stay grounded better coming out of corners and has better top end power.
I chose the two bikes accordingly due to the conditions they live in.