Ok, first, Burnt Valve? This is like saying the glass is half empty. It's an arguement that has gone on for ever. It's such a heated topic that it's as bad as talking about the gov policies. that being said. I'm not sure of your intentions, such as offroad mud bogger or just a good old loud exhaust. So I'll just go that route and evade any issue of smog and crap discussions.
I run a 5.7 300hp crate in my k1500. You need to remember that an engine is an air pump. the more fuel and air you can get in it and the more you can get out the back, the more power it will have. Now if your engine is TBI or Vortec you can remove the cat and muffler and put straight pipe on without issues. But because you are able to increase the outflow I would put a high flow Air Filter on as well. The computer will compensate timing and fuel air ratio accordingly as long as you only do these things.
If you have a carbed engine you can do the same as above and you may need to open the jets a little to increase fuel flow through the carb. Again this is for if you are going for more power.
But I guess to answer your original question. A cat and muffler has an amount back pressure. Opening up the exhaust on a stock 5.7 will actually give you appx 5-10 hp increase.