Just got my 6x10 up and running enough for a real ride. Lots of pesky issues with the bike itself. Top speed was right about 20 mph with 48v, pulling about 1000 watts. The 9x7 pulls about 1200 watts max on the stock kit controller and my ping 48v battery, doing about 27 mph. So it's a considerably slower motor for sure, and the wattage readings confirm what Live for Phyiscs said about the 6x10 winding having a bit less total copper, and therefore a bit less power.
I found it a bit anemic at 36v, but at 48v I'm very happy with the performance. More amps at 36v would help though, of course. I am running the stock ebikekit controller till I get some lipo. I found the motor perfect for slow riding at 10 mph down a singletrack trail. Very efficient on the battery compared to my previous dirtbike, that used a 5304 in 24" rim.
With a 40 amp, 72v controller I bet this motor will make a pretty nice dirt rider for faster trails. The trail I ride is built for 5-10 mph, and mostly fairly flat so at 48v it's just about a perfect match for the 6x10 on 48v 20 amps.