I've never had a problem with the TRX-5 lacking bottom end, so with that in mind you might want to try either a Sparks TT or a Sparks MX. I would say the TT would be a tad better than the ESR pipe on the bottom, although it is mainly a mid range pipe. The only problem is that Sparks does not produce their 2-stroke pipes any more. You are gonna laugh, but maybe try a Pro Circuit MX pipe. I had one on my Open A Trx 330 built by Tom Carlson at TC Racing, and it was an awesome combo. The only pipe that was better was my Sparks TT BB pipe, and they are nearly impossible to find now-a-days.
Teddy