Figuring in drivetrain losses, you were pretty darn close to that 240 mark at the crank.
I would agree. From the multiple chassis dyno information I have read, results can vary widely depending on the dyno. The only real data you can gather is multiple runs on the same dyno to see if thing improve.
I'm far from a turbo expert, but from some of the info & articles I have read, Gary is 100% correct. The closer you get to the edge of a compressor map for a given turbo, the less efficient it becomes. A turbo needs to be selected based on where you plan on running it. In the middle of the map is where you want to be. You want whatever turbo you use to be in it's " sweet spot" when you are trying to make max power. That 25-28 psi range is on the ragged edge of what a stock Garrett can do, and it's losing a lot of efficiency doing it...
Plus, going back to the original reason for this thread, it you're using a one piece, at 28 psi, you're probably pretty much stuffing all the air it can handle through it. That leads to inefficiencies of its own...