Im about to fully modify my twin RCMK engines, 15+hp, in a couple of 60"boats, and Im a little apprehensive with running the 1/4 flex that I currently do..... Should I upgrade to the 5/16th flexi to avoid breakage/failure? Who sells em??

Cheers Scoot.

There was a thread on JIMS where Dan P did a nice custom 5/16 shaft setup for JMatak's Apache twin, The shaft tube was water cooled. You would be best to wait JIMS to be back on line for the full story.
I do remember that CP mods was running a full mod twin with 1/4 shaft, with no reported issues but again shaft tube was water cooled.
I think Viper Hulls in OZ is a supplier of the 5/16 flex shaft and collets, and as mentioned DP customised a Ron Buck stinger., have a look on his you tube channel.
Good Luck with it!!

I'm running a 60" Sonic R60 with inline 26 modified Omen engines and running a 1/4" flex. Never yet to break or damage a shaft, But .... I am quite fussy about maintainance and oil ... works well for me.