How much horse power does a stock 300ex have???
In general pipes are pipes. Some are tuned to different RPM's, but they all give you about the same results.
If you were to put on a pro-flow or K&N filter, with a full exhaust, and jet the carb properly, I think regardless of which exhast you use, you can expect at least 5% gain, up to about 10% gain. Since you are starting with 18 hp, that only gives you about 1.5 hp... Now if you were to do all of the above, *and* put in a different cam, to take advantage of the better breathing system, you might see 20%, which would put you in the mid 20's.