Can the A/C temperature through a vent be significantly lower than in the air handler?
Low R22 pressure (presumably suction pressure) combined with correct superheat typically means that there is too much compressor capacity for the airflow.   This can happen when there is a clogged air filter or weak blower motor, or less often an undersized interior fan coil mismatched with a larger capacity condenser.   I have seen systems where, unbeknownst to the owner, there were two filters installed inline, one was at the return air grill in the building and that one was being serviced; the other was at the air handler in the attic, and that one had never been serviced or removed and was completely obstructed.

The most likely cause I can think of for colder air at “the vent” than at the air handler is the temperature across the evaporator coil is very uneven, say 34 degrees on one side and 50 degrees on the other side.  In some ductwork situations air supplying one set of registers comes primarily through one side of the evaporator coil while air to other registers comes primarily from the other side of the coil.  A proper trunk and branch system with taps near the air handler discharge could do this, for example.  Typically, an evaporator will be much colder on one side than the other if the system is low on refrigerant charge, because the refrigerant prematurely boils off completely before it crosses the coil.  Low charge will also usually cause low system suction pressure, but the superheat will also be notably high (typically 30F or more at the compressor instead of 15F).

My take on your post:


Low 40s at a register is too cold for a properly sized system.  High 40s is the safe lower limit for AC.  Low to mid 50s is typical.  Your system has either too little airflow or too much compressor.  Look for airflow restrictions or an improper blower speed setting first.  If in doubt, have someone measure the airflow.  Your system should supply roughly 400 CFM per 12000 BTU of compressor capacity.  For example, the air handler for a 48000 BTU system needs to push about 1600 CFM.
You cannot have a temperature at your supply vent that is lower than the temperature leaving the air handler.   You are misreading the temperature at the air handler.   If you are reading the temp at the suction line exiting the air handler, for instance, then the refrigerant inlet side of your evaporator coil could be much colder.
If you measured the air temperature leaving the air handler and had a warmer reading than one of the supply registers, then there is possibly a large temperature spread across your evaporator coil (30s-50s), and your system may be low on charge in addition to the airflow being too low for the cooling capacity.  Superheat would also be high in this case.   Have someone else recheck the superheat.  If in doubt, crosscheck this with the subcooling at the condenser.  The subcooling should be around 8F-12F.  Less than 8F is very likely low on charge.  If the system is not low on charge, then this inconsistency is simply a case of the temperature at the air handler being read inaccurately.