30+ degF of superheat is not uncommon with these cap tube driven systems. If you feel the need to push your flow rate, a txv will get the low side in the zone. Since you don't have sky-high discharge temperatures, you are not very undercharged. Remember, max efficiency may not be at max heat transfer rate. As you add charge, compressor current draw increases...the optimum for r290 is a little less charge relative to r22. Propane has less mass but more heat capacity. This means high superheat is not necessarily a bad thing. Keep on doing trials. I would rather slow down a fan than chop down a heat exchanger myself.
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