I can build 10 pumps the exact same way and none of them will be the same with max pressure, that's just how the 200 is. I am a bit surprised at the 300+ but the fluid is cold 70 or so , I bet after some driving the fluid thins out pump clearance opens up that will drop some.
I hear that. I've only had a gauge on one 200 and it was a Bowtie OD built trans in a BBC Chevelle. It was a good trans and would see about 220-230 psi. I really need to get a gauge on my current one and see where it's at. I have a feeling it's real high. The Powerglides we build in my shop will go around 275 for 2000-2500hp builds.