Yes, I believe it was modified maybe 15 years ago in pursuit of a solution for the perceived hinge-in-the-middle handling while under heavy race use by Tony Baker.
While the frame stiffening may have contributed to the overall stiffness, Mark Brown replaced the stock RZ500 swinger for a CBR400RR NS29 swinger when he took over custody of the the bike and it worked well.
He contemplated removing the couple extra pounds of angle iron welded to the frame rails but never got round to it - I am going to hack it all off now that I cut a piece out to accommodate the fairing brackets.
The ZXR7 front end, Brembo stoppers and NC29 swinger and Ohlins shock seem to work well together and to be honest, the placement of the frame stiffeners seemed arbitrary.
There are 2 frame rail braces under the tank that probably do more to stiffen the entire package than the bars down the side of the upper rails.
I have replaced the GSXR750 rear wheel with a Marchesini TZ250 unit that already shaved a few more pounds off the rear - the angle iron probably accounts for another 2 or 3 pounds, I have my eye on an aluminium 1KT tank to shave another pound or 2 off.
Eventually I will probably retrofit the entire bike into one of the RZV frames I have - that should be goo dfor another 5 pounds or so.