On the flip side, you should consider just how much line strength you really need. High-modulus line (such as Dyneema) may be cool, but we (myself included) often get suckered into using the stuff when it isn't really necessary. Hand-tensioned lines don't take enough load for stretch or strength to be an issue. Regular braided polyester is good enough. Plain old 3/16" Sta-Set, for example, has a tensile strength of 1400 lbs. Even if you cut the core and did a cover-to-cover end-to-end splice (like the one shown for the Marlowe line), halving it's strength the join, it still exceeds the working load limit of the (assuming 29mm) blocks.

For dinghy controls, I worry more about picking a line with a good grip in the cleat and a nice hand, than about the tensile strength.

Oh, and if you're happy with your exit blocks, fine, but for in-line exit applications, a block like http://www.harken.com/productdetail.aspx?id=5038&taxid=424 is generally more suitable.

Regards,
Eric