99 GT lost .2 with the Intense FWI (note this car also lost .2 with a DHP PCM). Also keep in mind this car is/was one of faster NA GPs. 14.2@96. This car has always done it best with a HAI.
97 GTP.. no noticeable difference between Intense FWI, SLP, Thrasher, HAI.
All were done on the same day at NRP (summitt raceway park now), at 90* temps.
As far as the extension, you don't need it for the Thrasher setup. The Intense FWI doesn't require the extension either, however you can relocate the IAT to the fender well for a minimal gain..