There is a ton of info on this if you actually cared to look. Certain cars have certain issues that are well known among owners groups, this is reflected on their respective owners forums. BMWs are notorious for this problem, as are VWs and Hyundais and its been discussed to death. There seems to be on the other hand very few complaints from actual Mazda owners on this even after 6 years of SA production. In fact its just the opposite for the most part with many cars at 100k miles or more and no issues. The few pictures I have seen do show some buildup but not enough to be relevant. Some (most) of it can be shown to be to related to driving the car short distances with insufficient time to get the temps up to where the carbon shedding can occur.
There is a particular vendor that claims to show how much oil gets through the PCV system using a catch can, oil that has been said to cause deposits on the intake valves. However, those pictures when closely examined show that the captured "oil" is mostly moisture with a tiny bit of oil on top....
So, just because you read something on the internet doesn't mean its real either.
As to any cleaning using water to dislodge the carbon, I would consider asking Mazda first. If the intake valves are indeed kept at 700°F or more when the engine is at proper operating temperature, hitting them with water might cause some stress issues. Bending, twisting and fracturing valves could get expensive fast.