I'll chime in...
I had an sti with a rear facing newborn. It worked but was somewhat uncomfortable. All rear facing click-in-with-a-base seats are uncomfortable in and "fun" sized car. They just are.
My son is now 4. I had him in the back of my e36 m3 with bucket seat and he did fine with that. He can't buckle himself in which meant I had to kneel in my seat to buckle him. I had him sitting in the middle rear of that car.
I now have him in the brz sitting behind the passenger's seat front facing. He's comfortable and it's only a little uncomfortable for my wife up front.
When/if we have another child I'm going to consider another car but probably end up keeping the brz. Here's why:
-There are smaller rear facing infant seats (like the coccoro combi). Not all of them are those huge click base models.
-The rear facing stage doesn't last forever. Before kids did you plan on keeping this car longer than 2 or 3 years?
-My 4 year old will soon be in a booster and be able to sit behind me comfortably. He should also be able to buckle himself soon (right now he just isn't strong enough to do Britax's buckle)
Seat I had in the sti was something like this: [ame="http://www.amazon.com/Graco-SnugRide-Click-Connect-Tangerine/dp/B00AHVR1QI/ref=sr_1_2?ie=UTF8&qid=1382469311&sr=8-2&keywords=rear+facing+child+seat+graco"]Amazon.com: Graco SnugRide Click Connect 35 Car Seat, Tangerine: Baby[/ame]
I had this in the sti and the brz: [ame="http://www.amazon.com/Britax-Marathon-70-G3-Convertible-Seat/dp/B00834D5SO/ref=sr_1_1?ie=UTF8&qid=1382469354&sr=8-1&keywords=britax+marathon"]Amazon.com: Britax Marathon 70-G3 Convertible Car Seat, Onyx: Baby[/ame]
This was the only seat that I could get to work in the m3:
http://www.target.com/p/combi-coccor...FUjZQgodyW8ABw
In the brz I now have: [ame="http://www.amazon.com/Britax-E9LH41A-Pioneer-Harness-2-Booster-Seat/dp/B00D6TI84W/ref=sr_1_9?ie=UTF8&qid=1382469423&sr=8-9&keywords=britax+pioneer+70"]Amazon.com: Britax E9LH41A Pioneer 70 Harness-2-Booster Car Seat - Onyx with Carrying Case: Baby[/ame] (my only complaint is that the headrest rattles when he isn't sitting in it. I found a thread on a car seat forum (yes they do exist) that says it has to do with the nylon material (for easy washing) that the headrest wings are made out of. I'm going to try foam or foam tape to quiet it down tonight.)