Taking it out the butterfly valve doesn't get you any HP at all. I think SCC did that on there project car and it showed no gains at all. But it just gives you that mean sound down that everybody loves. Adding in a K&N filter will give you like the others say a few extra HP, which is worth it.
The reason SRI
is still louder is because it eliminates all the factory tubing and replaces it with a much larger metal piping, which resonates and sounds a lot louder, plus you get more HP gain out of it.