The 850, 900, 950, and 1000 are all 90 cubic inches. The difference in HP was mostly from carbs & reed stops. From what I recall, the 100hp had an improved-design exhaust baffle that routed the exhaust pulses more efficiently. My old 850 had the older-style flat stainless baffle plate. When I rebuilt the powerhead I used a 100hp baffle & carbs but I don't recall changing the reed stops. The old Beast ran really strong, though, on a 14' Fiberform v-hulled runabout I got it up to 50 with an old, beat-up 2-bladed aluminum prop. With ski prop a solid 44 mph and would rip a 200-lb slalom skier right out of the water. That was a great motor!
The 99-c.i. 1150 is a completely different Beast and the 90-c.i. motors offer no comparison.
The 1100 was simply a 1000-design block with slightly larger bores for a nice boost in HP. From what I understand they run pretty good, too.
The 99-c.i. 90-hp distributor model had extremely small ports, with really good low-end. But they ran out of steam and didn't do much above 4500. So putting larger reed stops in that motor wouldn't accomplish much.
The first-gen 1150's had Humongous intake & exhaust ports, and Merc choked-down the hp by installing large restrictive venturis in the carb (with smaller main jets to go with), and very skinny reed stops. Conversely, that identical 1150 became a rip-snortin' 1350 with bigger reed stops, smaller, less restrictive carb venturis and fatter main jets. That was a nasty motor and one of my favorites!
Sounds like your engine was originally jetted for altitude and if you're now at sea level, it might run too lean. If you do ever switch to 100hp carbs, they should already have the correct main jets.
If you use the 1100 reed stops/blocks and 950 carbs, be sure to check jetting at full throttle (warm up, go full thottle for a time, then shut the engine down and check plugs). The spark plug insulators should be tan-ish, but if they're bone-white, too lean. Go up to the next jet size in that case.
HTH & Happy Rebuilding.............ed