In my mind the Stik seems to be made for flying at a slow or medium speed.
At the same RPM you'll most likely have less pulling power with a 13 compared to a 14.
The Stik is not a very 'slippery' design, so the 8 pitch might not gain a lot of speed over the 6.
Of course, you might bolt on the 13x8 and like it better than any other.
We all like different things when it comes to props.
My brother tried a Graupner 3 blade (not sure of size) on one of his 91s and swapped back to the 14x6 after two flights. I think the plane was his Super Sportster.