I mean from last summer with a built trans until now. I'm sure the increase in power has something to do with that as well
I wouldn't think you'd see increased trans temps from added power unless you were using the added power?
How "built" is the trans? Are you running an upgraded cooler and cooler lines?
600whp shouldn't tax a built trans...although your tq multiplication table might need to be slightly altered for it or maybe change to a different stall speed in the converter
But I'd start with an upgraded cooler if fresh fluid doesn't help, as mentioned above, built transmission's do run a bit hotter and thus the fluid should be changed a little more frequently, well IMHO, 2x as often as a stock trans. Especially if you're havin fun with it!
:2cents: