Have had my Zephyros 1 for three years and, yes, the lack of a bathtub groundsheet concerns me, although I've had no problems to date, it has remained 100% watertight.
Maybe I've just been luckier with the weather, though, for there were a few occasions with my old Ultimate Tramp 1, which did have a bathtub, when I woke in the small hours to feel pressure under the groundsheet and find it was flooded outside. While I've had hours of overnight rain in my Zephyros I just don't seem to have had the same torrential downpours as before, but I also like to think I'm now more experienced at choosing a pitch that is less likely to flood.
My memory is a bit hazy as to when and where, it was either on here or OM, but when I was thinking of getting a Zephyros, or maybe just after I got it, another owner assured me that he hadn't had a problem, even when there was water running underneath his tent.
As for the smaller porch of the Zephyros 2, neither of them is exactly spacious and I don't think you'd notice a lot of difference. I extend my space by holding the flysheet door open over a walking pole, which helps keep all but the worst rain off you provided you've pitched as WC recommend with the wind at the back of the tent.