The transom rating is based on the manufacturer's assessment of an engine's weight and overall speed range of the hull at the time it was built. Key-thing is not to exceed it which theoretically could invalidate your insurance.
My understanding is the DT100 is around 150kg. Depending on the actual transom rating you may be able to change it to 115hp if that's what the plate allows. Hopefully Osprey owners can confirm their experience of this, given the DT100 is an old engine now.
Back to propellers. Aluminium are great all-rounders. There's a misconception that going to stainless is going to reap huge performance benefits. Yes, on larger engines they do have the benefit with a stiffer blade construction if won't flex at WOT, and should fair better long-term. Drawback with stainless, is if you hit something that power isn't absorbed by the propeller and can result in stripping gears or even the splined drive shaft. Aluminium, then you're likely to bend the blades on any impact and save the gear-box, and in real terms they are fairly cost effective to replace or repair.
You'll probably need to remove the propeller to see what's on it just now (there will be marks on the back of the blade or potentially inside the hub). Lowering the pitch, can improve hole-shot at the expense of overall speed, however it will allow the engine to reach maximum rpm at slower speeds.
Key thing - is what's the boat achieving just now in terms of rpm at full-throttle, trimmed, etc. And what's your aim, increasing speed or hole-shot from a standing start?
Is that with or without VAT?