If you're wanting pure fun then get an Ultra Stick .60 for that .91. You can bang the sticks, bank and yank, and generally abuse the thing to your hearts content and it won't tip stall. Landings are a no brainer too, even with the extra weight.

I'm curious what would happen if I put a 16x4 prop on this plane with the OS FS91.

You'll have to add weight to the tail, which will increase the wing loading and make it difficult to land.