An infant seat is outgrown when the child reaches the weight limit or there is less than one inch of shell above the childs head.
you can move her to a convertible car seat in the REAR FACING position any time. My boys were both out of infant carriers by 4mos, but remained rear facing until 18mos and 35mos. An infant seat is no safer than a convertible car seat, but at her age she may enjoy a rear facing convertible more, since it sits more upright and she will be able to see out the side and back windows, and it will give her more legroom.
A pediatrician is great for your childs health, but will know nothing about child safety in the car. The AAP recommends children remain rear facing until at least 2yrs old and 30lbs, or when the child reaches the rear facing weight limit of the car seat. Most pediatricians still dont' quote the AAP, their own organization, correctly.
Car seats in the US will rear face to 35, 40, or 45lbs. Children can remain rear facing until they reach the rear facing weight limits of the convertible car seat or there is less than one inch of shell above the childs head. Feet touching the seat is not a problem, nor a reason to turn a child around forward facing.
So go for it, get her a convertible car seat and keep her rear facing. google Joels Journey and Extended rear facing for more information.
A few great seats are; Graco MyRide 65, Sunshine Kids Radian, First Years TrueFit, Evenflo Triumph Advance. All will allow you to keep your little one rear facing for a couple more years, and in a harness until at least age 5-7. :)