The under hood mods do transfer between the body styles, but when you go looking for a cat-back, suspension, or real brake upgrades, your choices get really limited. Heck, the cat-backs just don't exsist.
Suspension stuff is basically limited to high performance oem equipment like a GXP. Same with the brakes. But if serious modding isn't your thing (and there'* no shame in that) the Bonneville is still more than enough vehicle for the average person. I'm driving an 02 Monte Carlo SS daily, and when I get the chance to take Bonne out for a spin, it is truly a delicious experience. The added luxury and power are still intoxicating. I am dying for a long summer road trip to soak it all in.
2000 SSEi Galaxy Silver...All the toys: K&N,Gutted A/B, Mobil1.
2002 Monte Carlo SS: K&N, ZZP DP, Mobil1.
2001 Suburban 2500: 496 BBC K&N, Mobil1