Thanks for the advice Dave.
Shaggy, if I were you I'd also check how much available space you have on your transom and also check the weight of the aux. Not sure if you have a 4st or 2st but in the Yammie 2st range there is a jump in weight from the 8hp to the next one up (9.9hp) of the order of 9kg. The 15hp is the same weight as the 9.9hp. It doesn't sound much but it could be the straw that broke the camel's back!
If you do a search on 'auxiliary engine' you'll find several threads where members give actual figures, ie. what speed they make with their aux on their rig. You'll probably find at least one with a similar set up to you! HTH
Edit: a few old threads to have a look at: