I did some tests tonight.
I measured a (modulated) 670mW of 650 red from my scanners. Then I removed the reflect blue dichro (25mm Laserwave) and the red power shot up to a measured 800mW.
I then re installed the dichro and tried different angles:
45° = 670mW
60° = 685mW
70° = 705mW
80° = 760mW
88° = 790mW
I didn't fancy trying 90° for fear of killing my lasers, but a fairly conclusive test. I did try rotating the dichro through 360° in approx 45° increments, but it made no difference.
This; combined with my massive cube losses and galvo mirror overspill mean that my measured 1.8W (at 5v unmodulated) of red ends up as 900mW