Doing this post for anyone who is looking for any feedback on spintech. Just installed a spintech sportsman xl (3441) muffler with Dynomax intermediate and tail pipes. Also put in a K&N filter and replaced the 2" snorkel at the end of the filter housing with a piece of 3" rubber hose. I can feel a power increase from idle on up. It seems to really take off at 3000rpm.
A buddy has a 95 F150 with a 351 and a Gibson cat back. The spintech has the same tone, just a little louder.
The only complaints are its a little louder than what I was looking for, and the tail pipe exits a little too low. Definitely not a big deal.
Overall I'm very happy with the results.

 

I had spintechs on my mustang. They sounded great and were only $70 a muffler!! Not sure how much power it added b/c I put them on the 2nd day I had the car and never got on it w/ just the mufflers on it, but w/ a cold air and the mufflers the car ran 9.2 in the 1/8.

 

I just had Spintechs with the Dr. X put on my truck for a true dual exhaust. They have the Pro Street mufflers on and sound great! They have a much better sound than the Gibson Super Truck I had on before. I would also recommend them. Although, in my case, I would not suggest doing them youself.