Awesome question, particularly with the cost of R-22 continuing to go through the roof.
The most important part of any of the retrofit refrigerants (a refrigerant used in place of the OEM system desired refrigerant) is that they should be charged properly. This means they are to be added using one of 3 methods: Using either Superheat, Subcool or the Weigh in method of determining the correct amount of refrigerant. This includes the need to clean the coils and verify proper airflow before installing it, or any refrigerant for that matter into a system.
Since MO99 (R438) and many other refrigerants are not a complete blends they must leave the jug in a liquid state meaning the jug must be upside down. A charging adapter should then be used to ensure it is a vapor before getting to the compressor.
Operationally, I've seen it both increase and decrease capacity in a system. (Although to be honest that probably has more to do with how well the coil was cleaned and accurate the charge is than the specific refrigerant in use.) It is best to stick with a refrigerant once started as an alternate on a community as they are not to be mixed. (NEVER MIX ANY REFRIGERANT IN A SYSTEM.) This can be dangerous. In other words, if your community is going to stick with the R22 equipment, don't have some on your property be MO99, others R421, still more R407c etc... That situation can turn into a logistical and bookkeeping nightmare!