Ok, I'm going to admit I was wrong, at least up to a point on the brace height and difference in speed of the bow. I went to the local archery shop and used their coronagraph and here are the results.
I draw 27" and have a less than crisp release. The same arrow was used-540 grain 56# spine cedar shaft with large 5" feathers. Both bows used in the test were 68" Northern Mist Whispers.
59# bow 6 1/4" brace 164,162,166,163 fps
59# bow 7 1/2" brace 161,163,160,158,161 fps
The 59# bow has string silencers and is 18 strand B-55.
56# bow 6 3/4" brace 163,166,165 fps
56# bow 7 1/2" brace 164,166,164,166 fps.
The 56# bow doesn't have silencers and is 16 strand B-50.
For comparison, I shot a very r/d 69" 55# bow with an 18 strand B-50 string with silencers and shot 165 and 166 fps.
I couldn't really compare the Whisper to the Classic because I have skinny ff strings on the Classic.
I'm going to conclude on my bows the brace height doesn't make a noticeable difference on the bow speed.