I agree with Neil. Also, I don't recognize a size 10 Lewmar winch .. perhaps you've confused it with the 16? Nonetheless, the smallest primary winches I'd consider on a CD-30 would be 30-ST Lewmars and they are just adequate. I have Harken 32.2's on my CD-27 and the high power is used more often than the high speed.
Self tailing is a no brainer if you sail single or short handed (or if your sailing partners aren't trained gorillas). It's like having another crew member aboard
Some of many other things to consider is the use of the primary winches for things other than sheeting sails. Man overboard gear is much easier to use with a good winch as the prime force applicator. I have used my primary winches as a makeshift anchor windlass with the rode led through a snatch block on a stanchion base. It's about the only way a boat without an anchor windlass can kedge off successfully. Being a pretty heavy person, I've sometimes been winched up the mast with a big primary self-tailer because the size 6 or 7 mast winches aren't up to the task (There's a lesson here for those folks who cut their halyards just barely long enough). For all of these uses it is important to get a fair lead to the winch with a snatch block or genoa sheet block.
I realize that good winches eat up a significant number of boat units but they are more than worth it in the long run. When one is old and beat up like yours truly the increased cost seems a small price to pay for having the extra power available.
Hope this helps ............
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Montana Dispensary
I intend to replace the bronze, single speed winches on a '83 CD30 with single speed, self tailing winches and would like to know what size Lewmar would adequately replace the old Lewmar 10's that are now on the boat. I have the self-steering and furler and am finding that I need a self tailing winch for a more comfortable single handing.