Having just read Paul Adsett's latest dilemma over on Film tech, I'd like to assure him that it is barely noticeable.
In fact I'd go as far as to say, the only real way of detecting whether or not the apparatus is set to two or three blade shutter mode other than the obvious on screen brightness difference, is by placing a solitary fully extended finger onto the lamp house cover.
Then you can just about feel a very slight change in the vibration levels between the two settings but you need no anti vibration mat nor any other such device as it is such a subtle difference and noise levels from the machine remain unaffected.
I proved this is very much the case by recording a short section of video footage that captured the movement in a glass of water placed in close proximity to the projector running at 24fps in two blade shutter mode.
My findings were that the only real detectable movement from the water within the glass came as a result from sound vibrations from the internal loudspeakers.
As the piece of footage became almost silent at one point, there was no noticeable movement from the water just from any shutter imbalance.
See Video, Sam 3819
In the following link address for reassurance,...
As I said at the time, if you visualize the effects of a pair of training shoes placed into a washing machine at 1600rpm spin speed, well it's about 180 degrees apart from these side effects of less than perfect mechanical rotational balance.
That is just about how insignificant this imbalance actually is!
And for the benefit of Matthieu, very very simple and easy indeed to changeover for the models which can feasibly accept such a device.