i use a 45cm chinook skinny extension:
ive snapped a couple of skinny's on it and its still straight. Given the first breakage was on a 5.3 rigged on my 4m (so about 30-35cm of extension), and the wave destroyed every other part of my rig, thats not bad.
the second was a 4.7 so not so relevant.
I've used my new 360 skinny (my old 4m with an annoyingly shorter bottom section) twice with 45cm of extension on 5m and yet to break- hasnt had much punishment but.

I would just use the 45cm extension: masts are expensive and can break just as easy!
if u still have warranty on your skinny check you dont viod it by using max extension (more importantly make sure you meet the requirements for having enough extension in the mast) and go for gold!!!

Thanks guys, since a I'm a flyweight, the softer sail might work well for me anyway.

Regards, Harrow.