Finaly tried a mazdaspeed6 lightweight body (my previous ms6 body was regular weight), and found that I was a bit slower with it compared to the GX body. The ms6 was easier to drive and felt "smoother", likely due to softer turn-in, but the greater overall downforce of the GX results in better lap times for me.
If I had to rate the front and rear grip of different bodies, it would be something like this:
GX: F4 R4
MS6: F3 R3
LTCR: F4 R2
STC4 F5 R2
Blitz WRX: F2 R4