Maybe Adam (@Tune+) can jump in on this. He's said that the best DP for the stock turbo is the stock DP. I spoke to DHM's turner about a year ago and he said the same thing. My guess, and it's only that, is that there's a bit more to be found at the top end but that's after the turbo has fallen off the cliff and you've missed your shift point.
The FiST doesn't respond to DP upgrades as well as some other platforms, i.e. the stock DP isn't close to being a limiting factor. The general consensus around here is that, if you're staying with a stock turbo, a larger intercooler and a pro-tune will get practically are you're going to get power-wise. Further intake and exhaust upgrades add noise (and some of that noise is lovely) and itsy-bitsy gains.