Tig welding the blower housing may warp it. If you warp it, the rotors will gouge it, and then you're going to have more problems.

There's been threads on here, and other forums before about people welding the ports shut, and none of them proved any gains whatsoever.
Leaving well enough alone is better than dicking up a decent design for just going after a sound like the ricers go for.

Just my 2 cents.