I recommend either the 8hp or 9.9hp High Thrust outboard by Yamaha. Any additional HP will likely gain you nothing as you will never push the boat past hull speed. The high thrust models have the very large low pitch props and beefier gearcases.
I have the 9.9hp High Thrust on my 2520DV and can get about 6.5mph in perfect conditions, no tide or real wind.
I too have a 2310 and am going to add a kicker, my 9.9. Question is which bracket will extend far enough out so the lower unit doesn't hit the trim tab?
I am also looking for the strongest one as well, have seen a few of them brake while the boat is being towed.
Thanks
Sean