More efficient ac pump?

I haven't seen inside a C171, but perhaps you can find a youtube of a rebuild and compare to Sanden innards. My guess is that it has the same many double-acting pistons with swash plate, so should be as efficient. The only reason to switch to a Sanden might be for future availability or to have different ports. There is a long thread w/ many photos on AC compressor options for a slant-six. If you do go Sanden, the SD7 is newer and more common than the SD508 and has more pistons (smoother).