Sorry if this is a silly question with an obvious answer, but...

...if you put baffles in an aftermarket exhaust, will any power gains be lost?

 

Yes!

 

Thought so.

Arse.

 

Get some ear plugs

 

I've got em. Can't hear the radio then.

 

I've a straight through exhaust ..

Made 316bhp @ wheels "open".
Made 305bhp @ wheels "baffled".

 

Would this be the same on a NA car where back pressure is needed ?

(But I'd expect power loss as well)

 

I'd heard due to the way vtec cams have some degree of overlap, they don't like backpressure in the same way older engines did. The 4/2/1 manifold does all the real work on back pressure anyhow.

I think old carbed engines liked back pressure, but moderns don't.