I guess as a first step I would check the pressure nipple on the muffler to make sure that there actually is a hole in it. It could be that a bit of manufacturing debris is in it or that the hole was never completely drilled through.

I run the TT.46PRO with a Tower muffler and a 11 x 5 APC prop. I havn't tached it lately, but I did when I first put the Tower muffler on...... it boosted overall output about 5-600RPM, if I recall. I think it turns around 13,500, but as I said, I might not recall the exact numbers.

Anyway, it sounds to me like something may be wrong with your new muffler.