I haven't had time to post what I did, but I recently did a GXP suspension swap. Here'* what I found:
1. The GXP friont sway (FE5) is the same as the SSEi bar (FE3). 30mm...same PNs
2. The front struts are the same.
3. The front springs are a different P/N than the SSEis. Now that I have them installed, it seems my front end is sitting a little higher. I believe it'* because the GXP has the N* engine anb heavier trans. I'm not sure if the spring rates are different.
4. The GXP rear sway (FE5) is 22mm and hollow. You have to order new bushings. The brakets and endlinks are reusable. Good time to upgrade the endlinks to poly bushings. I could not find usable 22mm mid-point bushings. Just ordered stockers.
5. The rear springs are a different PN and seem to be a different spring rate.
6. The GXP rear shocks (FE5) are beefier than the SSEi (FE3) shocks. Seem to be better build quality.
FWIW - I didn't notice much difference between the two suspensions. I saw moderate improvement in cornering and on/off ramps on the highway.
If I were to do it all over again, I'd just do the GXP rear sway and rear shocks. I got the whole suspension setup from Ed from a parted out 05 GXP so I'm glad I didn't pay full price for new GM stuff.
Hope the helps.
**EDIT - I misposted originally and said the SSEi had FE2...it should of been FE3.**
**EDIT 2 - It might be FE2 afterall. Need to confirm. .**