Peter,
I used various Meade SCT's 8" through 12" for many years....the mirror slop was significant due to the crap mirror/ focuser connection.
I then moved to a C11. The focuser mechanism is completely different and much (much) more capable than the Meade.
When collimating, how much movement of a centre star do you see when focusing inward then outward???
This is a key to determine the mirror slop.
An easy way of reducing it, is to run the focuser through the full travel (about 30 turns) in and out a few times. This redistributes the grease between the baffle tube and the mirror support tube. Reduces clearance and reduces slop.
Out of necessity I used mirror locks on the Meade, but never needed them on the C11. I don't use an external SCT focuser, but have used and enjoyed a Feathertouch 10:1 micro focuser.
http://starlightinstruments.com/stor...product_id=261
HTH