You need the prop which will put the engine at the speed it likes to turn. For the Saito 150 (and I would guess the 180 would be the same) this is about 9k. For a YS 140 for example (and this is not the start of at YS v. the world comment...) they like low to mid 8k's, hence a bigger prop turning more slowly.

The YS 140, especially the dingo, is also more powerful than the saito 180, again another reason for a bigger prop.

It will probably be ok with a 16x8 but you would almost certainly get more thrust, as well as less noise, from the 17x8 prop mentioned above. For the bigger engines and especially four strokes, bhp is a rather misleading way of comparing set-ups. It comes down more to torque, so just because you put a larger prop turning more slowly you can get better performance despite being lower on the bhp curve.

James