Here is a nice writeup article about the Parra 2.5cc diesel engine in the Australian Control Line Modellers newsletter:
They got 16,300 RPMs with a APC 8x4 prop.
And a APC 7x6 prop yeilded 17,200 rpms
The special trimmed 7x3 prop gave 26,500 rpms.
it might do better with a good 6.5x6.5 or 6.5x7 prop too.
As a modern diesel competition engine it outperforms my old Quarter Midget racing glow engines by a considerable margin.