In my humble opinion mate, you'd be nuts to drill your Staintunes and I doubt it would do much anyway. They sound great the way they are and besides, you'd wreck them. Do you have a cat bypass fitted? That would make a bigger difference than drilling. If you have your heart set on drilling something get your hands on some stock pipes or TORs and mess with those. Just my 2 cents worth, mind. Best option would be to sell the Staintunes and put the money towards something that would give you the sound and performance you're after. If you want louder, Jardine might be the way to go (if you can get your hands on a set).
As for fuel consumption, I'm not sure what the direct impact would be but I do know that if you're anything like me you're going to want to hear the extra noise every chance you get so will be a lot heavier with the right hand = a lot more fuel used and probably rubber too.