The ComfortSport is NOT your only option for extended rear facing! The ComfortSport has a very low shell/low top harness slot, most kids outgrow this seat by 3/4yrs old, and it will only harness to 40lbs. There are much better options out there for the same or a bit more money.... but the extended use of the seat with hte harness makes it worth it. I wouldn't recommend the Graco Comfortsport. you daughter is smaller so she may be ok in it, but again for your money, there are much better options out there, I wouldn't purchase the Graco comfortsport.
There are many many seats that will keep your daughter rear facing to 33/35lbs, and then forward facing for longer as well!
The Evenflo Triumph ADVANCE will rear face to 35lbs, and forward to 50lbs. Its about $160 and its a GREAT seat, has a great depth and is very padded and comfy. Most kids can rear face in this seat to 2/3yrs old. I really like this seat, for the 35lbs rear facing weight limit, the 50lb forward facing limit, and the lower price tag, and of course the comfort. It will fit most kids up to age 5/6 in the harness forward facing.
The First Years TrueFit will rear face to 35lbs (with the head rest on) and to 65lbs forward facing. This is around $180, but it has rave reviews and everyone who has purchased one really likes it. I've only played with it in the store, but it fit my 3.5 yr old awesome. Most kids can rear face in this seat to 3+. It will fit most kids up to age 5/6+ in the harness.
SunshineKids Radian65 will rear face to 35lbs, and foward to 65lbs. This is a great narrow seat if you need to fit 3 across, or need the extra space in the car. Most kids can rear face in this seat to 3+ This is a steel seat, so its heavy and well made, and has an 8yr life span (most seats expire after 6yrs). This is $200, and it will fit most kids to age 6+ in the harness.
Britax Marathon (or Boulevard or Decathlon) will rear face to 35lbs, forward face to 65lbs. These seats are $250-$300, and are ultra comfy, the quality is outstanding. These will fit most kids to age 5/6 in the harness forward facing, and most kids can rear face in this seat to age 3+. My almost 7yr old just outgrew this seat, and my youngest was rear facing in it until 2.5 (he's tall and hit the weight limit) and is now forward facing in it.
Kids will outgrow a seat rear facing when there is less than one inch of seat above their head, or they go over the rear facing weight limit. the harness needs to be at or below the shoulders for rear facing, and at or above for forward facing.
Good luck in your car seat search, and good for you for following extended rear facing!