My biggest concern would be how much water you'd shove into your intake in a rain storm. Aside from soaking the filter, in a good downpour you'd be heading towards engine damage. You can't compress water like you can air.

 

Ram air most definiately helps a turbo car, probably even more so than in a NA car. Especially when your running close to the edge of the turbos efficiency.
Think about it, not only does it let the turbo spool faster/easier, it also gives lower turbo outlet temps, and creates less back pressure since the turbo doesn't have to spin as hard to make the same boost. You obviously cannot see much difference on a chassis dyno. I have added ram air to turbo cars in the past, and noticed significant improvements on the street/track, especially at high speed.
I am also looking into some sort of ram air system for my car, so far I am thinking an additional hood duct (like the VII) might be the best solution. How do you feel about cutting your hood? Somthing with a black lip to match the stock venturie duct and no body shop work required.
I am just concerned about fifth gear light load cruising could cause the MAF sensor to go crazy, so don't start cutting your hood just yet.