I respectfully disagree with Beenthere about the minimal SEER increase by adding a TXV valve. I indeed find that with those cheaper, very small condensor 10 SEER systems the efficiency gain is lower, but overall with a decent 10 SEER system you can expect at least 10% efficiency improvement and less wear on the compressor.
But, of course, adding a TXV valve is easily said. It takes quite some work and needs to be done properly by a trustworthy professional. If you only plan to keep your system for another year or two, don't bother looking into this. You'll never get your investment back.
Don't try to re-use an old TXV valve when you buy a new system. The TXV valve itself is not expensiver and most new systems come with it.