Like the PP, my 5.5 yo was rear-facing (RF) in the MA until 4 yrs 2 mos and is still in the MA FF. He has some torso growth left still, but I'm not sure off the top of my head how much. I don't see him growing out of this very soon, though
. He was RF in the Touriva (pre-cursor to the Scenera) until just shy of 4 yrs and then he got too tall, but he just grew out of it FF a few months ago (by weight). The Comfort Sport is a seat he was even with the top slots on when he was less than 3 years old (my friend had this seat and he rode with her a couple of times when we visited them).
Honestly, even if a child did have no time FF in a seat, due to RF for so long, that wouldn't bother me. I'd rather they are able to RF for as long as possible than to worry too much about how long they'll have FF in that same seat. To minimize how many seats you buy, however, you look for the seats that'll RF longer AND FF longer. My longer-torsoed baby moved into the MA (RF of course
) a month ago, just shy of 4mos old. I know that he has more time in this seat than a seat with a shorter shell, since he's got a long torso. That'll come into play when he's FF, too, though, so again, I have a taller seat that also has a higher-weight limit for FF. HTH and it's great to post questions like this, since many of us wouldn't think of it along those lines.
ETA that my dd (now 8) still fit in the MA by height until 7yrs 2 or 3 mos. She was around 45-46 lbs at that time. This would have been just under a year ago (she turned 8 in Oct). I don't know that ds2 (5.5yo) will get to be *that* old, since his growth patterns are different, but still, he definitely has some time left, where he would not, height-wise, in many other seats.