Two feet makes a big difference. The width is really what matters though. I have a 1648 and, to me, it is the narrowest a boat should be if you are going to be on any decent sized body of water. Years ago a friend of my Grandpa's invited us to stay in his trailer on Reelfoot and use his boat. Not exactly sure what the width was but it was a 16 foot jon and much more narrow than a 48. The thing was very tippy. Three of us fished out of my Grandpa's old Lowe 1648 no problem without concerns about moving too much. In this boat if one of us shifted too much everyone felt it and reacted accordingly, which caused the boat to tip in the opposite direction. So all day we tipped back and forth. Felt very much like a canoe. A wider footprint on the water is much more comfortable to be in. If you fish with kids...get a wider boat.
I can't speak specifically about the boat you mentioned with the split center seat, but I did mod mine to split the seat and it was the best thing I did. Opens things up so much and since I use the boat for duck hunting too it is so much easier to get around it in waders. I have my two deep cycle batteries in the two separate seats, which helps distribute the weight.
As far as the 15 horse on a 16 goes, my grandpa always had a 9.9 on his 1648. We fished a few lakes that had 10 horse limits, so it was often the only option. We didn't break any speed records but it got three of us to where we needed to go. He also had a 20 horse that he threw on from time to time and it felt like we were flying then! I think you will be ok on planing out, but I just about guarantee you will be looking to upgrade. When I bought my boat it had a 1971 Johnson 25 on it. It moved the boat pretty well, but it was about on it's last leg. Last year I bought a 2002 4 stroke Yamaha 25. I am hoping that it is a propeller issue, but it doesn't push the boat nearly as well. I kind of wish I had held out for a 30 or a 40 now.
