Dan,
You are correct, the Nissan's are really great engines. Four years ago I too bought a Niisan 9.8 for my Ariel which replaced a 9.9 Merc. It was quite a learning experience, leaving Merc. and going to Nissan, and a pleasant one I might add. I did find the 9.8 just a bit tight in my hull 391 ob well. The 9.8 Nissan is the same exact power head as the 15 Nissan and a minor modification easily turns it into a true 15 H.P. My dealer was very happy to work with me and I did go down in size to the 8 H.P. which we use to this day. It has never given a one bit of trouble. We get a new impeller every other fall, change the lower unit every fall and fog it with fogging oil till it stalls for the winter. It really does move our Ariel very well!
Jim