18SF planing speed and performance questions

[erebus]

After running the boat a little more frequently after last years rehab, I have a few performance questions, which I thought I'd run by the hive mind.

The boat is a 1975 18' SF.
Last year I put a new 40 gallon fuel tank below decks, all the way forward in the coffin, put the batteries (2X group 24 ) as far forward in the console as possible, and moved the console itself forward from original location about 8 inches. I think the trim is good, as she sits right on her lines, fully loaded.

After removing a ton of rotten wood out of the transom and putting a new coosa core in, and swapping the motor for a 1998 Mercury 115 2 stroke, that came off a 20' seacraft, I was wondering after this past weekends trip across the bay to Wellfleet if the prop etc. should/could be changed/modified.

She doesn't seem to get up on plane until about 13-15 knots and that requires about 3800-4200 rpms.
Once up on plane I can take it back to about 3600 and she'll hum along at about 18-19 knots.
Any less rpms, and she starts to fall off.
Really starts to fall off at about 16 knots.

That's running with the (mostly) stock boat, an AL leaning post, some beach crap, me and the wife (me big, she small) a full tank (40 gallons) 2 anchors, and some miscellaneous tools, oil, boat junk etc.
Oh and a bimini, in the up position. Heres a pic.

Does this all seem in keeping with how she should perform?

Unfortunately the prop is an unmarked aftermarket job, but I'm guessing it's a 13 3/4 X 15.

Any thoughts?