I have a 159SW and now we have two kids, the baby's Pebble seat goes behind me but without a base. I'm 5'10" and the seat has to come forward one notch further than I'd like. My wife is 5'3" on a good day, and I don't think the seat would go in on it's base behind her in the passenger seat. If it does, it'll be *very* tight.
We have a base permanently fitted in her C-Max and there's plenty of room even if I'm driving, but then I think the front seats are thinner and there's probably an inch more legroom to start with. It's not the most exciting thing around, but that C-Max really is a terrific family car.
Do you really need the base? Belted up properly, the difference in safety is merely academic, and it only takes 20 seconds to reach over and fit the belt around it. It's no big deal. The base is just quicker to get the seat in or out.