If you want a gruntier and tough Severne sail look at the Blade or Gator; the S1's typically have less bottom end and are built lighter. The blades have full xply windows but the S1's have monofilm. Sails usually bust around the clew/window so the reinforcement is here is pretty critical if your sailing logo or punchy waves regularly, which goes for all sails.
Unless you have a mega budget try and keep your whole setup on the same mast (eg 400), the 5.7 Blade will probably have more power than the 5.9 S1 and it'll just fit on a 400 with a 45cm extension (2011 has 442 luff). This is heading toward the limit of your extension but they're cheaper to replace than masts and you don't have to worry about carrying 2x 400 and 2x 430 or whatever if you go on a trip. Just have a 30cm Carbon and a 45 Alu.
BTW From my experience Alu bases are the go for waves, the weight difference bet carbon and alu is minimal, but you can depend on the alu bases not to leave you high and dry 500km from the nearest shop.
I think that your biggest sail is usually the most important in your quiver. You'll have more fun if your the one planing whilst others are dogging when its light, and when its marginal.. your heading out and a big set approaches you want the power to get out of the ****.
http://www.severnesails.com/2011/products/sail_blade.phpwww.seabreeze.com.au/classifieds/default.aspxEDIT:
Just re-read your post. Even with a bigger sail your ~90kg on a 85 is going to be pushing it! Modern boards are very responsive at bigger volumes you might find your money would be better spent on a ~95L wave board. This might sting you more than updating your sail quiver but usually if its light you not doing mega jumps so it'll last heaps longer than a high wind board.
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