Hopefully someone who is a car seat technician will see the post and be able to respond better, but I'll give it a shot with my limited knowledge.
When rearfacing with a convertible seat, the seat is usually installed at an angle which will allow more room at the foot of the car seat. Often times one will need to see some tightly rolled towels or pool noodles to get the correct recline. So, many times the seat is not reclined enough, so there is not as much leg room. Note, that the recline is only crucial when the baby has trouble with keeping neck upright. Once their neck muscles are more developed, the recline may make the fit more comfortable by allowing more room at the feet. However, as the child develops and can keep their neck more upright, the recline is a parental decision and not a necessity.
My 3 year old is average height (and didn't rearface nearly long enough), so I never had a problem with not enough room. I've heard that many babies do not seem to mind the limited leg room (even though it often bothers the parents). I'm not sure what I would do if it distressed my little one. I'd probably take the seat to be checked by a car seat technician or try out other convertible seats to see if I could find one that gave him more room. As cold hearted as it sounds, the lack of leg room is only a comfort issue and I'd now be reluctant to turn him around before 1 year AND 20 pounds. Even if his neck were strong, his bones are NOT developed enough to withstand a strong crash! This could cause major, major damage and leave you with broken hearted. However, as a mother it is really difficult to purposely place your child in a position where they are comfortable, so I'd hope to find another seat that fit better.
Again, I'm hoping a tech or someone more familiar than me will read this post, because I'm sure they have much better reasons/ideas.
Hope this helps,
Jennifer