I have removed the inboard Atomic 4 from my boat,
and have 'repowered' with a 6HP long shaft 4-stroke outboard.
I gleefully removed every last vestige of the inboard,
likely totaling 500lbs. Removing the engine,fuel tank, exhaust system,
wiring harness, starting battery, etc... opened up a lot of usable space
in the boat. I removed the prop shaft and strut, and glassed and faired the holes,
as well as the seawater intake, eliminating two places for water to enter the boat.
The 55lb outboard pushes the boat every bit as fast as the inboard did,
while using 1/3 the amount of fuel. The outboard can be steered with a long
'tiller' on the throttle which helps in low speed maneuvering.
The sailing performance of the boat has increased dramatically,
especially in light air.
It can be tricky getting the boat up to speed in a choppy sea state,
but as soon as it is up to speed it will carry though some rough conditions
without too much trouble. The 15HP should power you just fine.