It won't be a noticable loss in power, we are only talking 3-4 hp. You're butt dyno won't be able to tell the difference there.
Heat shields very rarely form a good seal with the bonnet.
If its performance you want, then getting a MY06 OEM airbox is the way to go. Or get yours modified by the venturi stack mod that rtbiscuit offers in the members trader area of the forum.
I had induction kit fitted simply because the looks and sounds far outweighed the few ponies I lost.