Using 190mm I saw a 15% improvement.
Update January 2013:
I have been riding with Zencranks PAS 190mm (so far on my chrono bike) achieving victories and podium placements at Italian Championship level Senior. I recently tested the new integrated Zenpedals, and the accelleration from seated is even better, very good from changing from flat to uphill for example in sudden switches you do not feel the longer crank and it's easy to maintain the cadence, when switching down gear remains constant; the sensation in compare to standard 175mm is to use less energy and reduce fatigue at same speed. On flat riding the tendency is to pedal around 105 rpm even with a 55 chain ring. My personal opinion is cadence may increase with Qrings (Rotor). In short uphills when seated is easy pedalling at 85/90 rpm. I plan to test soon the 184mm with new Zenpedals on my road bike, that with the old non integrated pedals made my legs feel "stiff" on the distance.
Gabriele Davi
Piedmont Amateur Team