I use a similar method, except that the tight briefs I use on top of the jockstrap aren't UnderArmor brand. I've tried hitting it lightly a few times, and it does hurt a bit from the sudden pressure, however light, so a hard kick from someone will definitely hurt. I'm just glad they encourage kicking above the hips during practice here in my dojo.
In some ways, having a cup is better than nothing. I haven't had any accidents with or without it so far (though there were some close calls a few times when I wasn't using a cup==:)... and considering what's been mentioned, I pray that I won't ever have to.
What's your opinion on the one-piece groin protectors compared to the two-piece ones? Do you think it's safer to use the single-piece ones? If that's the case, I'm thinking of getting another one - the Adidas one from Karate Depot, perhaps.