It wouldn't be worth the money on a trainer. You'll get more performance increase by just playing around with props to find the one that works best.

A tuned pipe will increase RPM, but only at full throttle. It also makes the engine's throttle response very non-linear, the engine will suddenly increase RPM as it comes on the pipe.

The FP engines aren't timed to really take advantage of a pipe either, so the RPM increase won't be huge.

The biggest advantage you'll notice is that with a full length pipe that will likely extend past the wing, your plane will stay a lot cleaner on an average flying day.