flyinpete used a OS .15 on his Ultimate. You should check with him when you get your .15 installed on your plane. I would think a .074 (equal to a .10)would be the minimum you need for power, so it is probably too underpowered to really tell what it needs. I would use the CG called for in the instuctions book. Then watch out for your roll rates with the .15, I would use what the book calls for at first. Here is flyinpete's thread.
http://www.rcuniverse.com/forum/Watt...1611850/tm.htm