Yeah your calculations seems spot on and man is your system performing incredibly well. I'm jealous because I would be more than happy to get even 55F at 2000rpm.You need to know what your ambient temperature over the condenser and your relative humidity are. I like using a wet bulb to directly measure it. Without knowing those two values it is impossible to know if your system is falling outside of the designed cooling output.
I just checked my Sanden 4261 equipped single evaporator system as a reference point for this, 84F and 55% RH here today.
A probably overly mathematical approach, but if you look at the chart in the FSM there are no points for 84F and 55% RH, I thus averaged the values between 50% & 60% RH and 80F and 90F.
50% RH, 80F = 53F
60% RH, 80F = 56F
Average of those two points 54.5F
50% RH, 90F = 59F
60% RH, 90F = 63F
Average of those two is 61F.
54.5 + 61 = 115.5 / 2 = 57.75 or call it 58F.
This chart is at 2,000 rpm, center vent temps with R12.
With R134A it is
50% RH, 80F = 56F
60% RH, 80F = 59F
Average of those two points is 57.5F
50% RH, 90F = 63F
60% RH, 90F = 66F
Average of those two points is 64.5F
Average both and get 61F at 2,000 rpm center vent.
R152a falls somewhere between R12 and R134a in performance. R134a being slightly worse than R12 or R152a especially as ambient temperatures increase.
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Thus my unit cooling at 48F at idle is pretty darn good, better than GM considered acceptable performance per repair guidelines. At 2,000 rpm though, the system will cool to ~34F before the cycling switch calls it quits, that is the very edge of where I can make this system cool before the evaporator core turns into a brick of ice. i recently replaced the cycling switch, it was acting up and holding the compressor on long enough to get the vent temps in the mid 20s despite it being over 90F ambients and freezing the evaporator core up. Even with the freeze up being the limiting factor, it is out cooling GMs minimum standards by ~22F in my ambient temperature and humidity based on the original R12 performance expectation just goes to show how much better modern condenser designs work as well as my upgraded cooling fan moving more airflow.
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Which is the dual air condenser that you recommend because I haven't found a compatible one that measures over 30"?