I wouldnt go with a stainless prop on a sib, or any non performance boat for that matter. It wont give you any speed or acceleration advantage and it might even make the boat worse than it is now. With your engine a stainless wont run faster, the difference in blade thickness will be marginal and the extra weight of the stainless may actually make the engine run slower.
More importantly you are taking away the last line of defence for your gearbox. Touching the bottom will happen at some point, with an aluminium prop its not a problem. The prop will bend or break and you can get it repaired for £30. With stainless, the prop wont bend, which means something else has to, this can be the gear box, drive shafts etc. It now becomes a very expensive repair.
I know stainless props look shiny and sexy, but after many years of chasing performance gains with lots of different props, I have come to the conclusion that the standard ali prop ( + or - 2" pitch to allow for loading variations) is always the best solution on a leisure boat.