Update:
I got a Diablo chip from 2D's Performance in LA (
http://www.super-chip.com/ ). $250 including overnight Saturday delivery. Delivery charges were about $35. Great customer service, no BS, and no hard sell.
Butt-O-Meter says somewhere around 10% HP increase. Shifting with agressive throttle is a lot closer to redline than stock. The normal slightly rough and lean idle popping is gone, too.
I figure it's worth it for an AOD tranny, maybe not for a stick with later models. If I still had the '98 GT AOD, I'd put one in in a heartbeat.
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2K GT Stock
FoolRichard