I'm currently running Small Blocks Fr and Rr. Just experimenting since I got them cheap. It's not a good choice of tire for most NJ area trails. They are fast rolling and light. They are NOT very grippy in corners or under heavy breaking. I rode them for the first time at 6-Mile on Saturday. I found myself skidding the rear and not able to hold my expected line often. I had to brake far sooner than normal to take corners. They did roll crazy fast though.
Why on the rear only? There is a school of thought that the back tire doesn't need to be as robust because there is more weight on the rear tire to aid in traction. The front tire has to do both braking and steering with less weight so it needs to be more aggresive. I don't follow this school but the theory makes sense.
I ran the nev front/ sb8 combo for awhile. Makes for a good compromise between speed and grip, but I ditched it in favor of a more balanced setup. The sb8 skates around on anything slick or wet like it is made of lard.
Running the sb8 up front IMO doesnt work anywhere with roots or fast corners. Six-mile for instance in anything less than perfectly dry conditions is scary with a sb8 up front.
Funny that you mention the combo though, I noticed it on a few bikes in this months various mtb mags.