Yep, need to replace the indoor coil.
Sometime ago, a chart wa floating around this site. It was from Bristol compressor.
They tested a 13 SEER condenser withan old 10 SEER coil. They could not get the efficiency, or the rated BTU's from the mismatch.
It isn't a matter of replacing the indoor coil OR the TXV. You have to replace both to get the rated efficiency. You also could have operating problems, although not as much with straight air. Depending on your location, operating costs, cost of equipment, amount of time before you sell the home, you might want to consider a higher SEER.