The answer to that is: "A definite Maybe"

There are exhaust deflectors on the market, and some people suffer no RPM loss, when using them while others do.

One time, I rigged up something for my son's trainer plane that was 1/2" thin-walled aluminum tubing that the smaller muffler exhaust (About 3/8" ) slid into with lots of room to spare. The tube was clamped to the side of the fuse and ran all the way back to the tail, yet it had no effect on the RPM, yet another time, I used a 3" commecial deflector and had a significant loss of RPM.