The SMK will be slower for the same powder charge. The short bearing surface creates less pressure than the longer bearing surface so to bring the pressure up to the same the 230SMK will need more powder. This has been my experience testing between the 230SMK and the 230 OTM. I wonder if your barrel is a true 1:8 twist? My gun likes the 230SMKs better than the 230 OTMs. Not sure why that is.
I am sorry to hear the 230SMK are not working for you. Like I said my gun likes them better.
How far off the lands were you running the 230SMK?
I agree 6" @ 900 is nothing to complain about! I would be happy.