Welcome to ribnet!
...... "underpowered", is all relative.
My old SR4 had a 25 on the back. I bet there's nobody here would say it was big enough, but it could hold a steady 21 knots, the speed that anyone I cruised with would normally cruise at. Yeah, 'twas a tad lethargic out the hole, but once on the plane was fine 2 up.
If you want eye watering speed & "crew over the transom" acceleration, you'll want more horses, but for cruising / general use a 90 on a 5.5 should be fine.