How many miles on it? How good is the maintenence history (if you know it)?
Sounds like a small tuning issue, probably running a bit rich if you're getting afterfire. Also an exhaust with significantly larger piping will take a bit out of your low end.
Or perhaps this is all psychological since you used to have a GN;) I hear you there, my first car ('67 mustang) ran 12.2, second one ran 13.2 (WRX), now a 240 that might run 16.2 (

cry;) ). I miss going fast