I'm sure it must matter how far inboard/outboard the spin sheet block is placed. On the T it is always at the tramp lacing points or directly to the inner gunwales. But, the T is a 10' wide boat. I believe I20's (8.5' beam) sheet from the outer gunwales.
In general terms, I would think anytime your creating an excessive curve of the clew inboard toward the sheet point, you're loosing performance (too much "belly" in the sail shape).