Just getting a chance to reply to the rest of the thread now...
The Parkway fits my 6yr old 45lb dd the same way. The shoulder belt fit I have absolutely no issues with, but the lap belt fit I'm not 100% happy with. I break it down like this - if a parent came to see me with the seat already in use, I wouldn't recommend replacement. I don't consider it to be a bad fit. However, if a parent wanted to know if I'd recommend it to purchase, I would say no.
The Monterey fits my daughter beautifully - and contrary to the discussions that sometimes happen on the board here, for being tall and skinny she has no issues.
Truth be told, the Parkway SG has a lapbelt fit the same as the Nautilus in our car. I'd be inclined to use it as a booster before buying another seat if I wasn't going to get a better belt fit - but the Monterey is such a great fit on her, that I just can't settle for anything less than perfect and we'll probably end up with a monterey for her in the car as well.
So far as age debate goes, this is something that I would consider strongly - above and beyond maturity. I had read that children's iliac crests (sp?) on their hips don't really develop more until around age 5 or 6. Now that my daughter is older, I can feel the difference in her hip structure - I always give my kids "rides" before bed where I left them up above me using my feet on their hips to support them. My son who is 2 months shy of 4yrs old is harder to hold on - there is no natural place that his hip bones rest on my feet. My 6yr old on the other hand has a definite area that makes it very easy to support her by her hips. I actually commented to my dh about the profound difference and saying how I understood now why the iliac crests really are very important for a booster rider. I suspect they can be the difference between submarining and not submarining.
That's Canada's assertion, isn't it? I vaguely remember it, but can't remember the source. Not sure it applies to seats with anti submarine clips (?)
There weren't any seats with anti-submarine clips when the comments were made, and I know that while I haven't seen the data, I have had discussion with my instructor-trainer who works within TC, and she has talked about kids under 40lbs being at high risk of ejection, partial ejection, or submarining. (I think you may be able to find some comments directly from her in the CDN forum through searching her user id - allport
Of course, her comments are based on her own testing and experience - I don't know that it's based on any study, though only she could answer that one. Perhaps I'll email her.)