catted 2.5 isnt really much of an upgrade for a GTP either... if you have other mechnaical issues the extra air isnt helping anything. The 3.8 pulley is on a stock GTP because it wont cause much damage when you have other issues.

I've seen a few stock GTPs have fuel issues to cause 14:1 afrs at wide open... bad pumps, dirty injectors, bad maf, etc