Havent tried the 15M but unless your local is regularly down round the 10>12 kts level I dont think it would be necessary to go that big. The 13M is doing everything comfortably at 13kts & going well even below that.
Saying that though there may not be a problem with the speed of the 15M anyhow ? My 13M Rev turns almost (but not quite) as fast as my 10M turbo did (which is for sale btw

see buy & sell)