My opinion may be biased because I own a GXP and have never driven a ssei, but I love the power. Yea it could use some more hp but its still nice to drive, very smooth. Where I live there aren't to many car'* that I pull up to and think that I couldn't take them if I wanted to. And as far as mods, the GXP was only built for 2 years and limited numbers, if it had been out for as long as the ssei there might be more mods for it.
by the way I always wanted a ssei growing up. I almost got one before I seen the GXP. the only reason I got a GXP was because you don't seen them very often so people are amazed that it is a Bonneville with a N* v8
2004 Crimson Tintcoat GXP. Mods include K&N drop in filter future mods include nothing because they dont make any more mods
2005 Grey Chevy Equinox (wife'* car)
2001 Pink Barbie Jeep Wrangler (daughter'* car)
2006 Light/Dark Grey Graco Stroller (son'* car)