Hmm,
I can set my NC mirror mounted deflectors so heat isn't fully flushed out, and I can set 'em so the heat is flushed out, so I'm not sure what's occurring.
Did you have a Madstad on the GL 1800? If not, don't replicate what you did on the GL and expect 'similar' occurrences with your Madstad - equipped 6... Not because the 'Stad is bad, but because the wind flows a bit-different,,,
Not sure that there is a huge difference in the mirrors, but the shelter / cowling by the rads on a GL is wider/more flared than the F6B
Do you have highway pegs or a ragin' case of bow-leg???