I have been experimenting with impellers this weekend.
a solas concord 12/15 3mm cutback, and a solas concord 13/16 3mm cutback ( with the "race finish" - looks like a skat finish-
12/15 has MUCH better bottom end.
I honestly dont think my little piped 650s can pull that 13/16.
they spin the 12/15 with no problems though.
possibly with a larger engine you could enjoy the 13/16, I however am on the hunt to trade it for a 12/15, or a 12/17 SKAT.
nozzle boring makes a difference too, get your stock nozzle bored to 85 or 86mm this made a noticeble difference on my bottem snap as well.