I'm on a 2006 1800SR3 currently. with the 4.3 MPI which revs more freely than the carb'd version - which do you have?
I just had a custom 20" pitch high five done for me as my latest mod. 19" gives great hole shot and allows you to ballast the boat but yeah.. it is a little low so you need to watch your revs. 23" is pretty much not worth the cash whilst for me 21' was not quite enough power gain - though it is for most.
I've messed with pretty much every setup now and have fallen for the 20" but the engine can over rev if pushed... good balance for me though. all depends on what you're after!