That "average" includes the supercharged 03-04 Cobras, and the GT-500s, both of which will get 30, 40, 50 horsepower from a good tune. So that kinda skews the results a bit.
I would expect 10-15 hp at best. Although if you have a good amount of mods on the car already, it would probably get a little more. And if you have an auto, it is well worth it. Makes it shift way better. The programmer was the game changer on my car when I was full bolt on naturally aspirated.
Now that AM is paired up with Bama, if I were getting a programmer, it would definitely be from them. Free tunes for life from a reputable tuner: now that's pretty awesome.
What exactly are your mods, Imo, I wouldn't go screwing w/your ECU/CPU if your car is running fine, different cars react differently to the same tune. Sneaky98gt has a lot of mods that probaly had to tune when he was N/A. Its very possibly a safe tune can retard the way you ECU thinks and throw a code that is not warranted.
Do your search on these type of programs, there are plenty here w/ people prematurely changing the programs on there cars then the car is throwing codes!
I had my stock 02 GT in the shop just after I bought it for: 3.73 gears, Flowmaster catback, Pro 5.0 shifter and SCT chip. I really bought the chip to correct the speedo, but here are my results, with only those mods listed above:
Before: 217 HP
After 221 HP
(I've since totaled the 02 GT and apparently thrown away the dyno graph and I don't recall the TQ #s, but the gains were similar - very minor)
Hardly worth the investment while stock or close to stock.
I have SCT tuners for both my 98 GT and my 09 GT500. The HP/Torque gains on the GT500 were HUGE with just CAI and tune (50-60hp), but with N/A 98 GT the hp gains were minimal at best.
The most noticeable difference in 98 GT is the frimness of shifts with automatic transmission. This made a pretty noticeable difference in daily driving, but did not result in large HP improvement.
That being said, for just a little bit more money you can have 3.73 or 4.10 gears installed. This will make a big difference in the seat of the pants feel of your ride.