I also have a 100th + percentile kid. My suggestions, in order of personal preference and for a good fit in a Passat, for you would be:
-Clek Foonf - pricey and currently has a 43 inch RF height limit but easy to install, doesn't take up a ton of room front to back, and RFs to 50 lbs for heavier kids. I actually got one for $325 on an Amazon deal so good deals can be found.
- Graco MySize/Headwise 70 - I have one installed RF passenger side of a tiny '04 Ford Focus hatchback with a very decent amount of room for the front passenger. I HATED this seat when I first got one because the chest clip is stiff and the crotch buckle has a "trick" to it. In fact, I returned the first one I had in favor of the Foonf for a different vehicle because the person using the Foonf has arthritic hands and I couldn't see him unbuckling this thing. I still don't think anyone with weak or problem hands would have an easy time, but I do now have a MySize 70 as a spare seat in my mom's car. After working with the buckle for several minutes and figuring out the sweet spots, I have come to find it tolerable. I am not sure I'd use it as an everyday seat, but I would probably get used to the buckle if I had to. You can't beat the price, and it's the tallest RF seat you can get in the U.S.
-Diono Radian - People seem to either love it or hate it. I have read too much about the harness straps coming loose on their own, the company has a bad reputation, and the seat is HEAVY. But with the angle adjuster, it takes up very little front to back space, is very tall and narrow, and has a high RF limit. I liked the fabrics and found the buckles to be very easy to operate.