Perhaps the knock sensors are detecting a knock (which is not audible at higer rpm's) and the computer is backing off timing and boost to compensate.

I'm sure you know you're playing with fire. I would suggest setting it to the max allowed by Nissan (using a strobe light) and leave it alone before something expensive breaks. I found a slightly noticible improvement going from 15 to 17 degrees (spec is 13-17 for my 2000 non-sc.)