So far I've tried a few intensity settings out with surprising results:

I started with -i15 because I just assumed -i16 would bring my system to its knees, but hsrminer crashes with illegal memory access error at line 1500 with this setting. So I tried -i14; same result. Then -i13 and -i12; again, same result. Perplexed, I tried the other end of the scale and set -i1. The miner successfully starts and shares are accepted. When the first hashrate report prints it says 1275 kH/s. Thinking this might just be because difficulty is so low during the first minute or so of operation on a vardiff port, I let it run for a few more minutes and the average climbs to around 1290 kH/s.

I'm not sure what is going on here, but it definitely wasn't what I expected...  


EDIT - after ~15 minutes at -i2 average hashrate is 1298 kH/s, so slightly higher. Power consumption crept up by 10W as well. This seems to be going in the expected direction now.