OP, I'm curious on that 70-120 Nm rating you mentioned for the factory emtb's. I'm no guru on ebikes, but I work part time as a mechanic at a shop that sells Trek, Specialized, and Santa Cruz. I've gotten to ride Trek and Specialized emtb's, and they are impressive. However, I thought their torque numbers were usually in the 65-85 Nm range at max. Grantmac is correct in that they can demand higher wattage than the normally rated 250 watts in certain situations, but I'm not sure how much...noticeably higher than the 250 they are generally rated at.
Now, on to your question about the torque of a BBSHD, from material on them from quite a few sources they supposedly have at least 125 Nm. That's a substantial overall increase at even the higher 85 Nm rating on some of the factory motors. Those factory torque sensing motors are super efficient, however, and seem to get all their power to the ground excellently. I would like to see a Bafang Ultra equipped emtb in a head-to-head comparison, however, as it is torque sensing motor also and is designed to be integrated into the bike frame.
Even working at a shop, I too thought the higher end factory emtb's were too expensive. I recently modded my '08 Santa Cruz Nomad with a BBSHD and am thoroughly enjoying it. My Nomad has high end suspension already along with other quality components. After reprogramming the software in the BBSHD, I'm pretty impressed with how it performs. You can get smooth power delivery with excellent acceleration and good motor shutdown so you don't get a bunch of overrun like it had with the stock setting. With a good hydraulic brake switch on the handlebar, a shift sensor, and a good display, this setup can deliver very good performance on a real trail with actual challenging terrain. I am not at all disappointed with the results, and it's regenerated my thrill for mountain biking. At some point, one of the guys I work with will take our shop's Trek Rail 7 demo bike out to the trail, and we will obviously do some drag racing and on-trail comparisons. On the drag race I don't have much doubt that my bike will prevail. On the trail it will be interesting to see the comparison as we will trade out occasionally. We'll see how it goes. So far I'm loving this BBSHD equipped Nomad.