Pegasus,
Your pressures displayed in the photos look pretty darn good for a system retrofitted with propane. Have you measured your heat transfer? From your description, it looks like your evaporator us doing well. But what about the condenser? Your high side pressure looks good for a domestic water heater or air-cooled condenser. Depending on the conditions in the condenser, you may or may not stand much to gain.
Remember that r22 and r290 are pretty much polar opposites except for their similar pt curves. Where r22 does not fare well with superheat, with r290 a little extra is not a bad thing. Where r22 and oil do not mix well, r290 and oil mix like co2 and water. Running the superheat too low could fill your evaporator with frothy foamy blend that doesn't transfer heat well at all. If the evaporator cannot digest it all, you're in for a nasty surprise. Don't ask me how I know, proofs are overrated and such.
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