Billy think it comes down to cash mate !!! If your willing to spend on "performance" and have a trailer full of gear. you will need 10 board 7 masts 15 sails an 4 booms ;-)

I have made the mistake to often of purchasing sails that aren't compliant with my masts lengths and i have given them away (lucky they were second hand) Don't purchase anything with-out and adjustable masthead.Depending on the level of windsurfing and budget you are running id sygest not to worry about the mast flex. (this is for advanced sailors.)
I have successfully placed a 4.0 - 6.0 on a 430 and sailed comfortably (after adjusting harness lines) i don't have to budget to worry about mast curbs.
Anything above 6.0 i run a 490 and use sails adjustable 7.0+ plus.
All comes down to budget and how anal you are about performance. Rigging smaller sails is difficult because its hard to move harness lines in high wind (get tired quickly). Alot of advance sailors have smaller booms new mast and sail pre setup for dictating condition These sailors may go kilometers off shore..
You should put your weight and board size as this is a contributing factor to sail choice.
Depending on how far you plan on going offshore and level of fitness to swim-back dictates the distance of extension The higher the extension the higher the risk of gear failure. This simple equation can be can be implemented for mast flex.
The further you push the equipment (mast flex / extensions) the greater risk of gear failure.