converter slipping
That's just about where mine went, right at 700 increase. I know it made a huge difference in the way the car drove. It was an absolute dog down low, it in no way compares to your power plant, but I would have to think the same theory applies.
I dunno, if that's the only change you made, and it wasn't doing it before....
Maybe there's a certain point where that converter locks up for part throttle, so to speak, and you're just below it.