I performed some back-to-back tests on my ES after getting some feedback from a professional test pilot.  He said that the strakes(as far as I know there is only one commercial source for these at the moment) were "necessary" on the IVPT, "important" on the IVP and "made an improvement" on the ES.  I found that directional stability(feet on the floor, so the rudder was free to weathervane) was improved about 30%.  There was an increase in the yaw natural frequency and an increase in the yaw damping, both by about that amount.  After an aileron roll input it was also about 30% faster to enter a stabilized turn.  It was good enough for me to install them, although there was about a 2-knot drop in speed, excessive in my opinion - strakes like that should not measurable affect drag.  I think the problem is in the "toe-in" of the strakes.  They are geometrically parallel, but not with respect to the direction of airflow, where they are actually toed in.  Take a look at the strakes on Lears or Piaggios and you can see the difference.  Anyway, the parts from Mike Custard were of very high quality, they worked and I'm satisfied, but if I were doing it again I'd probably design my own.