Adrian,
I just came back to see what my RPM readings were and saw your post.
It seems a little odd that I was getting 13K with a 12x8 MAS and homemade inverted muffler while you were getting the same 13K with an 11X8 APC and TUNED PIPE.
The 10x8 sure does spin though!

One of these days I'll try the 13x7 APC I've got in the basement.