I have friends who have used notched shafts. Didn't seem to matter much. My favorite shafts are the Rob Allen 7mm Hawaiian sling shafts. They are awesome, but according to mike at florida freedivers, were a special order item and are no longer available. Sucks b/c I am on my last one.
FF divers now sells a KJ shaft that is similar, but not the same. They are decent, but I will miss my RA when some fish hauls ass with it.
I have used all sorts, from pretty floppy to stout and find that the 7mm is a good all around size for speed, distance, and hitting power. If I need a second shaft in a rocked up fish, sometimes I will use the 8mm to apply a little more persuasion on extraction.