Honeslty, maybe 1-2 at the wheels...flows sound good, but dont make any pwr by themselves. Need a catback at the least...a mid pipe is even better. Ive dynoed 18+rwhp with a slp x/loudmouth and 27rwhp with added ported/polished t-body/plenum combo. There will be none believers about this, but ive dynoed my car several times (dynojet) and those are honest #'s. I currently dyno btwn 241-243rwhp and 280+rwtrq.
As Big T stated above, don't expect to see any real power gains out of weld in flows. If you want to make big power out of the exhaust, then two main places to make it are the headers and mid-pipe. A full exhuast setup, consisting of long tubes, mid-pipe change, and full-length catback is usually 20-30rwhp and similar numbers in torque untuned for most n/a GT's. Naturally, if you get a catalytic mid-pipe, the cats are going to rob you of a few ponies up to, and of course a h-pipe isn't going to produce quite the same power up top as compared to an x-pipe. It all depends on what kind of combo you are using.
Big T, I believe the dyno numbers you got. Those are very possible and real world numbers. I believe it was RacerGT(Jason) who produced around 260rwhp in a n/a bolt-on automatic GT. So, I know its possible.
According to some people just welding in flowmaster mufflers make you lose horsepower Like as mentioned above mufflers will do very little for hp gains, were as headers and mid-pipe will give much bigger gains.
There is hardly anything wrong with the flow quality of today's aftermarket midpipes that are equipped with catalytic converters. Such losses, if they do occur, won't be a factor until you are making a LOT of power ( ie - read, high horsepower/boosted applications). Only in those high horsepower applications would you be "robbed" of a few measly ponies when comparing an aftermarket off-road pipe to an aftermarket catted pipe.