KWIL, most of the guys I know with 2011 WRXs who've been tuned by Tim end up with a 16-17.5 psi DD map, even with a DP. I think the issue he's concerned about is the stock I/C - the stocker is a 2 piece clamshell, the end tanks are clip fastened, and the unit can blow apart really, really easy. Put a solid TMIC on it and you could push 19-20 psi with no problem whatsoever with the right tune and fuel.
You're right, however, about his methodology. Thye push to see how far the motor can go safely, then back it down and fine tune for the final map(s). The last tune I got on my '04 with meth got up to 421whp, and we ended up at 405 - the timng was backed off touch and the AFR was bumped a touch for safety, but the boost remained at 25 psi. Out on the road, I could feel no difference whatsoever.