The 13/18 won't give you better bottom, but it will be a better "all around" impeller. I had a 13/18 on my 550 and my buddy was running a 15 on his 550. Both skis with exact same mods. His had better snap right off idle but I found it didn't have much mid range and definitely no top end. I like my 13/18 better. My buddy ended up selling the 15 for a 13/18.
You can always put a 13/18 in it and if you want a little more bottom end, have your exit nozzle enlarged a bit. That's what I ended up doing on my old 550.