I'm in the process of building a MCP for a TT GP .15 myself.

In order to 'tune' the set-up you want to run the engine at full RPM without any muffler and tach it. Once you put on the pipe it should exceed that RPM. You 'tune' it by gradually trimming 1/8" - 1/4" off of the header length until it stops gaining RPM. Typically, a MCP will net ~5-15% increase over an open exhaust when properly 'tuned'. You'll end up going past the peak RPM when you trim it the final time so that is why I like to trim only 1/8" at a time (I use a tubing cutter and clean the inside burr with a sharp pocket knife)) The best header length varies on one MCP set up to another depending on the size of the can, etc., so it is a 'trial and error' method.

Yea, this takes time and effort...but this is what I like about this hobby...tinkering!