Low mph in 1/4, any help?

[91LX2Bfast]

Well, all the 99+ GT's around here trap between 96-100mph stock, so maybe its an elevation thing.
I am comparing my borther's car's performance on two equally temp/humdity days. The chip was programmed with the LMAF and exhaust in mind. The 22C plugs are 1 step colder plugs to prevent detonation with the full advanced timing in the chip. Car runs 93 octane. Its proven that an o/r X-pipe is worth up to 20 HP at 6000rpm on a stock GT. Now thats not peak HP, but it's HP none the less. It should be worth almost 1 mph on a stock car.
ET has dropped, or at least will drop into the 13's with traction. But I am concerned with the MPH. Like I said, an equally equipped auto GT was running 99.5-100 mph all day with an exhaust, Diablochip, and MAF. So I know it wasn't the weather.