If you get a 40mm diameter suppressor it probably won't effect accuracy. I could see lesser diameter causing a few problems (maybe....possibly).
In the bottom pic, you can see that the bast of gas would hit the foam and frame of the suppressor. This could cause riccochet of the gas to hit the bb's in some odd way.
I would say, the larger the diameter, the less effect the blast riccochet would have on the BB...
The foam would absorb some of it, but with a smaller diameter, it would have less absorbant benefits. The blast would penetrate through the foam quicker.
Larger diameter suppressors would have thicker foam.
Regardless, im sure there wouldnt be too much of a difference with one and one without a suppressor.
Maybe im just thinking too hard....