The reason you can't get 30 more horsepower just from reprogramming the chip in any way is because a new computer set-up for the car in no way changes the engines out-put. The most you could do with the chips in cars today is adjust when the car shifts up to the next gear with an auto tranny, and things of that sort. To actually get more power from your engine you would need to add HARDWARE to the car like a turbo/super charger which forces the engine to eat more gas and take in more air, thus making the ngine work more efficiently and more powerfully. If it were just a matter or adjusting some program in the car, then you could see Dodge Neon's driving around at 250hp.
There is the possibility that the engine is capable of outputting more power than the programming is restricting it too, but I don't think that is the case with the GTO. Sure the GTO may be capable of putting out 410 hp, but the manufacturer (GM) puts a restriction on the engine in this way so that the car does not work itself to death and your engine doesn't blow out on you within the first 20,000 miles.
Basically, it's possible that the guy can reprogram the chip, but there is no way I can think of that you would get 30 hp out of it. If the GTO were able to put out 430hp easily enough, im sure Pontiac would make it do so. For the amount of extra hp you do get out of it though it isn't worth the amount of damage that would be done to your engine from pushing it soo hard.
EDIT - And I noticed we were actually talking about a 2004 LS1 goat here, which the same stuff applies really. I'm quite sure though that the LS1 engine is capped at 350hp and has been for all the years it has been out and used, so what the guy is saying is basically bull.