Re: tallest rfing carseat
Honestly, and I'm not recommending anything here, but if I had a long-legged child who had lots of room above his or her head in the CA, I would probably be willing to overlook the 40" overall height limit.
Size limits have a lot to do with the sizes and weights and heights of crash test dummies. The rules which govern which crash test dummy has to pass with which seat based on what the stated limits are, are convoluted and confusing. I can't even begin to summarize it accurately here. But basically if there's a dummy that is, say (pulling numbers out of my hat here), 41" tall and weighs 55 lbs, a car seat that allows for a 41" crash test dummy has to pass testing even though the dummy is 15 lbs over the weight limit. I do not know the actual sizes and weights of the dummies used to test the seats, but I can see why manufacturers sometimes put seemingly arbitrary limits on car seats which may or may not translate into this or that real-world kid.
As it happens, my daughter is longer in the torso than in the legs, and just making a guess based on how tall she is in the CA right now at 36" tall, my guess is she'll be pretty close to "actually" outgrowing the seat when she hits 40" anyway. So my current plan is to go ahead and abide by the 40" rule. But if I had a leggy kid I think I'd feel OK ignoring it.