Yep, as mentioned, its the difficulty in tuning them.
An SSEi will need a few mods to catch up to a GXP. Just look at the numbers. The GXP will run mid 14'* between 14.4 and 14.6 all day long, and an SSEi stock will run low 15'*; high 14'* on a very good day.
Now if that one company in Canada that was working on a GXP PCM that they said they were able to unlock does come out with something, we should be able to see this Northstar pull some more impressive numbers.
2005 Pontiac Bonneville GXP - For Sale ----- 1995 Buick Regal GS(X) - L67 madness
2000 Buick Regal GSE - Stock...for now
----- 1987 Jaguar XJ-* V12 - 12>8