The gun isn't near as gasey as some would have you believe. Its there no doubt but nothing I really even notice. I can't tell you what adding a suppressor would do.
I've never heard it was an issue nor would I suspect it would be suppressed. Shooting suppressed is a whole different ball game when it comes to gas in the face from a number of weapons. Reports seem to indicate it is particularly bad on the MPX. I had heard it from a few people and the more I research it the more I hear about it.