This is not a big issue, more in the nuisance category (if this is your biggest problem, car is in pretty good shape), but it sounds fairly common to Superformance cars. I have SPF #2895. SPF #2896 reports the same problem. The SPF owners who emailed fix information must have had the same problem.
Behind the turn signal lever is a Phillips head screw. Loosen that, then adjust the signal lever assembly left/right to best position. My lever was at the 3 o'clock position. Recommended position is closer to 2 o'clock. When positioned correctly, retighten the screw. (One owner recommended replacing the Phillips head screw with a bolt, not sure that is necessary.) I experimented a few times, adjusted left/right, drove and tested signals, adjusted again. (On my SPF, there is not much play in the electrical wires that come from the engine compartment into this steering wheel component, so I was limited in how far left I could move the signal lever assembly.) After final adjustment, I rechecked the finger, ear, lug adjustment as described at
http://www.craigscobras.com/tips/blinker/index.htm All of this substantially improved my right signal premature cancellation issue. It's still a little twitchy (cancels with small amount of steering wheel left turn), but is a lot better.