IMHO, I disagree with the advise on using a dyno to help tune the car. A "mustang" dyno is about as close to real world driving conditions as you can get, but even that is still not the same as real road driving conditions. How often are you going at WOT in 5th gear on the road when you're driving the car? This is a rhetorical question, the answer is not often, maybe 10% of the time, 20% for those in denial

. You want a tune that strikes a balance between normal driving (done most of the time) and hauling ass. You may get what looks good on the computer when you're on the dyno doing WOT runs, but the conditions are different on the road, and your tune will end up being off, and potentially risky for the motor. Also, no dyno on the planet can simulate the same load that's put on your motor when driving it on the road, so unless your tuner is good at making assumptions and swagging what the real load will be and dial in the right timing, you're motor's at risk when you stomp on it on the road.
My recommendation is to hook up a scan tool, such as EASE, and drive the car for at least 30 minutes around town, and include at least 3 WOT runs to triple digit speeds. Send your results to your tuner, and he'll be able to cook up a baseline tune that should get you close. Repeat once or twice to tweak it and you should have a good tune that gives you a nice idle, good normal driving, and good WOT without risking any damage to the motor. Yes, this process will cost you more money, but if you want to play... This also accounts for the variable fact that no 2 engines are exactly the same, and your tune is truly customized to your motor.
A good tune will gain you much more HP (and overall responsiveness) than what you might think. I'd venture to say so long as the injectors are staying below 80% duty cycle at WOT, and you have good intake air flow and temps, a good tuner can probably get you more HP than bolting on a set of headers/exhausts.
Again, just my $.02. Ok, flame suit is on now, give it to me
Jason
Viper GTS
Corvette ZR1