We tried a couple of different brands of 15x6 and all of them got around 9,300. The two different 14x8s gave 9,200 for one and 9,150 for the other. Our vertical test was to run about 300' level and pull max elevator into a vertical line and count. With the heavy airplane, vertical isn't forever. The 15x6s went up for a count of just under 10. The 14x8s went for a count of about 8.

If you use Dave Gierke's method of estimating thrust (load) the 14x8s would put about 15% more load on the engine we were using.