Got two RB433AH's on the bench. CM9 in each. Simple bridge with WDS setup as per the wiki.

Doing a 5ghz-Turbo mode I can get about 52.1mbps UPD flat line all day long
Doing a 5ghz-Turbo mode I can get about 42.6mbps TCP flat line all day long

if i enable N-Stream with bestfit/3200 I loose throughput and it goes down to around 15mbps and is very sporadic. Both boards are 3.24 and the BTEST sever is a 3rd board so as not to stress the AP.

Is N-stream broken? I used it a long time about back in the 3.13 days and was getting 37mbps UDP on a 20mhz channel so I know it should be doing more.

nstreme, real traffic (no btest) and on CM9

Not sure how you did the test. But make sure neither endpoints of the nstreme link is generating traffic for the speedtest. The bandwidth test is very cpu intensive. Nstreme by its nature is more cpu dependent. This would cause the test results to be skewed.

play around with the framer policy and framer limit.
for example, try exact size of 3999.

I also wonder what exactly "on the bench" is.
Did you check that your signal level is not too high/low/noisy?

tried using wirelest-test package?