I also have an Impreza (06 wagon) but I own it and I'm hoping not to have to trade it in. I'm determined to search all options.
Right now I have an old Evenflo convertable (needs to be replaced) for my 1 yr old, a Costco car seat for my 4 yr old, and a baby on the way. There is about 14" between the two seats.
I got a Fisher Price (made by
Britax) Safe Voyage
booster which the 4 year old will use when the baby comes. This
booster supposedly only occupies 13.5".