In our case, yes. My DS is almost 35mos and 2 mos ago he was complaining about his legs "hurting" when he was in his seat (I know he wasn't in pain, but that's the best way he could explain to us that he was uncomfortable). And we noticed this after a growth spurt left all of his pants too short, so I know his legs grew, not his torso (and I didn't need to move his harness straps up). As soon as we switched him FF, he was fine. No more complaining about his legs. My 7.5yo could still happily ride RF- she doesn't mind being twisted up like a pretzel, LOL, but my 5yo complained when I turned her back RF a few months ago- she said that her legs kept falling asleep and feeling tingly. I do think that the vehicle and car seat have a lot to do with it- the Radian left my DS with much more room to put his legs, while the BV really had him up against the vehicle seat back without much wiggle room at all.
Maybe my kids are weird, though, LOL!