Yes there is the Low Compression Head and the High Compression Head and mild modifications over the
years that's the jest of it. The TD type glow plug will have a knurling ring around the top. The low compression
heads have no knurling marks just smooth on top. The TD High Compression head will give you a good jump
in rpm.

2 head shims with a stock glow plug and a 6x3 prop sounds about right with 25% nitro.
Did it run steady with the back plate pressure? Agree with Chuck you should fly it and
enjoy! It may loosen up a bit in the air and I would imagine up to 23k unloaded is possible.

Post a vid if you can = )