Thing you have think about with a 20inch rim is that there is very little sidewall. So getting flex out of it is difficult. What will benefit you is actually filling the tires up with more air to get a better contact patch with the tire.
Just for giggles next time I would say no lower than 28 in the rears. I would even start at 32. Get them nice and hot with a Smokey burnout and give em a shot.
Most guys running anything under 20psi are on smaller wheels ... between 15-18 inch wheels. Due to the tall sidewall they get on the tire.
But lower profile tires wont benefit the same way.
sent from my galaxy s3. that would explain the errors.