Take it from me… the owner of gauran-fucking-teed the largest 3-piece iForged wheels on this site… or 3-piece of any type for that matter.
ITS NOT THAT BIG OF A DEAL.
I have driven hundreds of miles in downpours with these wheels at up to 100 MPH (yeah, not the smartest move in a downpour)
The vibration is minimal at best, and trust me, it can hold a lot of water in there. When sitting, you could easily fit 4-6 ounces before it flows out.
And there seems to be some confusion here. There is an indent in the inner wheel barrel forming another lip further in that is used when making the wheel (supposedly). It stands about 1/2 inch high, all the way around. The water collects between this ridge and the actual 3-peice mating surface where all of the bolts and flanges are. Roughly, on my front 11" wide wheels, it is about 2.5" in from the outside of the wheel to the inside of the mating flange, and the distance from the inside of the wheel to the inside of the rear ridge is about 4", leaving a good ~6" of area for the water to collect. My rears have roughly the same area even though they are wider as the outer lip makes up the additional 2".