Feet don't matter when it comes to outgrowing a car seat. Believe it or not, your child will be perfectly safe RF w/ feet over the edge of the seat. (Not that you were mentioning moving FF, but legs are actually broken more often FF than RF.) Especially w/ the Britax's antirebound bar, I wouldn't be worried about legs.
Your seat will be outgrown when your child reaches 22# OR the top of her head is less than 1" from the top edge of the car seat shell. She'll be safe in the seat until then.
Of course, if you don't want to use the infant seat anymore, you can always move to a RF convertible, as long as she fits properly (harness at or below shoulders, able to get it tight enough, etc.).