You should easily reach 50mph. I can hit 40 knots (46mph) with my 250 zuke. I'm not one for speed so I usually putt around at cruising speed 28-30knots happy as a lark. That motor is really sweet.
As far as price goes you have to figure in what you can get for the 115's and deduct that from the 17K I really don't know what they are worth (new 8K) but they are a very desirable motor and you will not have any trouble selling them at a fair price. You will spend at least another 1500 in rigging or you can keep the current rigging you have if the 300 will match to that. I know they have the fly by wire now for the 300 and that would be nice to have.
You will need the 30 inch shaft if you have a twin flotation bracket.
Filling the holes will be easy. Just grind a bevel in the glass around the holes about an inch or two wider then the holes. Get a wooden dowel that will fit into the holes. Cut it slightly smaller then the length of the holes, coat it with epoxy/cabosil and insert it into the hole. Cut some round pieces of matt and fill in the area till it is close to level with the outside of the bracket then fair over the matt, prime and paint.
If you wait till August you can get a deal on the last years motors. I paid 12995 for my zuke. The dealers around here wanted 17K for one.....which is probably why I still dont see any.
http://www.bayracermarine.com/
strick