$300 for one seat, or for 2? It makes a big difference.
Most convertibles have a 35 pound limit. Not enough of a difference from 33 to buy a new seat for 2 pounds, IMO. So you're left with the 40 pound seats. Right now, there are 3. A few months ago, there were none. Since you have 8 pounds to go, I would wait a bit, keep an eye on his weight, and see what else comes to market.
A quick rundown of the 3 current 40 pound seats: Graco MyRide, Sunshine Kids Radian, Complete Air. The MyRide has a shell approximately the same height as your Marathon, maybe an inch shorter. I'm not sure how the top harness slots compare. 65 pound weight limit on the harness, but like the Marathon, it will be outgrown by height before any child gets there. $150
Radian: Shell is much taller than the MyRide. 65 or 80 pound limit on the harness, depending on model, though kids will outgrow by height around 60 pounds usually. Can be more difficult to install than most seats, particularly the forward facing seatbelt install, but once you learn which tricks to use it's easy. Because of the tall shell and rear facing foot that you're required to use, it takes up a lot of room front to back in the car. I have mine in the center seat of my Civic because that way the top of the seat comes up over and between the front seats. Folds for travel and you can attach straps to turn it into a backpack. $190-260 depending on model.
CompleteAir: The shell is again very tall, but has a 40 inch limit for rear facing. I don't know as much about this seat as it is very new. $250.