A very good answer, indeed... A properly done performance test will include
checking for any air flow obstructions through the condenser and ensure that
the blend air door is traversing fully from one extreme to the other. Fan
operation will also be tested during this procedure. Observing pressures and
clutch cycling times and comparing these to ambient conditions (both in
regards to temp AND humidity). If a definitive answer cannot be reached
after this test, a recover and recharge is in order. All decent recovery
machines will report both how much refrigerant was removed from the system
(be sure that the tech knows about the ever present R-134 "burp" - more on
that in the next paragraph) as well as how much oil escaped during the
recovery process.

The "burp".... even after the recovery machine pulls the system to vacuum,
we must wait for several minutes before opening the system or performing the
deep vacuum process. Nearly 100% of the time, there will be a sudden surge
in system pressure. The most plausible cause that I have heard is that R134a
will become entrained in the remaining oil as well as the dessicant in the
receiver/drier (R134a is an extremely small molecule, I'm told - hence the
need for barrier hoses and special O-rings when compared to R12). To get a
true reading of the weight of the refrigerant recovered, we need to weigh
the "burp" as well as our first batch.

Once we know how much refrigerant was recovered, we can utilize all the info
we have gathered to come up with our repair strategy.

Increasing the volumetric capability of the compressor without increasing
anything else in the system makes no sense. The orofice tube is sized for
the system... presenting more liquid 134 to the orofice tube will only
increae the head pressure and result in compressor overheating rather than
increased cooling (though some incresed cooling MAY be noticed... until the
compressor pukes - not a cheap fix if you expect warranty on the repair).

While reputable shops will charge more for a repair, the actual cost is
usually less since repeat failures and misdiagnosis are far less prevalent
and are usually handled in a much more responsible manner.