Very impressive numbers! Yeah, I would epoxy some triangle stock behind the firewall. And, go over all glue joints; make sure you have the wing center section wrapped with glass cloth, etc.

I put a pipe on my Super Tigre .40 Bluehead. It's already turning a 10x6 at 14,111 with a stock muffler:

http://www.rcuniverse.com/forum/fb.asp?m=8181884

With the pipe I expect to break 15,000. It might be interesting to lengthen the header and see what it does with an 11x6, too.

I had to send my Canon camera in for service, so this is a poor quality cell phone photo.

David