Just read this at mobileunderground in the chefs corner:
Re: 6.5.x | 21686 - 21916 - 22047 - 23152 - 23569 - 23699 - 24635 - 25028 - 28244 - 2
After quite some benchmarking with ljankok, we found that the 216xx branch is nearly as fast as the original COM2 branch. Also, the official 231xx is quite slow...
Also, I have seen people over at DFT cooking with 21689 already.
Originally Posted by hd2owner 
i prefer 21687. seems faster and it is newer aku than 29009. 6.5.3.12 (23152) seems to be slower than 6.5.3.11 aku's.