Well, they basically do, the 750 and 600 frames are almost identical in the GSXR frinstance, as are the suspension and shocks. Going back a wee bit further, the GSXR 7/11 really did work brilliantly, light and it handled and didn't bend too much. And the Hornet 6/9s are still better than the Hornet 900... R6/R1 and R7/R1 were reasonably succesful as well, and I'm sure I've seen Kwak Z750s with the full fat 900 motor in them. But those days are gone for the big sports bikes, I reckon, think you're right there. Partly because the manufacturers have finally caught up
The 06 R6 weighed less than an RVF400.
You don't have to lose ability though, that's not what I said, the biggest single weight loss you'll ever do to a bike is the exhaust. The subframes almost certainly will be able to be reduced without any issues, mine would still carry a pillion if I wanted it to and it carries luggage despite the lightened frame. Likewise the front one and other general brackets, none of the weight loss I've done actually impairs the bike at all and the TL's probably very similiar. The TL starts out with an ally rear frame though if I recall correctly.