yeah, repeating what I read by manufacturing and on the fan sites... like you I slowed mine down when I had it and never had any issues like misbalance.. they just recommend for some reason to downsize your fan and run it at a higher speed then to turn it to low.. hhmmm.
I love using Infinity PC style fan.. found some pretty cool ones with built in speed control switch... it's framed in a
heavy metal frame for good stability.. and super quiet.. Budmiser uses a 6" for his exhaust fan.. hehe