A lot depends on your mainsheet system. On my F16 Taipan 4.9 I have an 8:1 Ratchamatic System. It works great and used 5/16" line.
However, I switched to 1/4" and love it. With 8:1 it is not hard to sheet or hold and boy does it go out when you want it to.
Another option is using larger line and tapering it down to a core that goes through the blocks.
Our
www.onlinemarinestore.com just found a really good guy that does the mainsheet tapering.
That way you can go with 5/16" or 3/8" and taper the core to really small stuff that goes through the blocks.
You see, most of the new stuff has UV protection for the core. Best bet it Marlow Dyneema Racing 2, or Samson Warpspeed, or New England Endurabraid.
You never want to go larger than 3/8" for a small cat though.
Even on my Wave that came with 3/8" I now use 1/4" and it works superbly -- easier to ease going around the A Mark, tacking, and on reaches, yet not a problem sheeting in with a 4:1
