Now I must say I have never thought of adding a whistle at the exhaust. Sounds interesting though (pardon the pun).

I think you will see a drop off in performance, huge is debatable depending on your engine and pipe configuration. Technically you're forcing the exhaust yet through another chamber which could mean too much back pressure choking out the engine. Also spent oil could plug up the whistle quickly.

But hey, this is a hobby and people try things. Give it a shot. I'm curious if the whistle will actually work this way.