Has anyone replaced the r-22 ( or from the start ) in a system with r-417
for either the environmental reasons or the improved COP
If so have there been any issues either with charging or operation

That must be a new type of Freon.

What would come OEM in a tranquiity 27?

relatively new 2002 roughly , hasn't been widely accepted , mostly because DuPont doesn't own it
likely R-410a

What are its operating pressures--similar to R-22? And what type of oil? Same as R-22?

http://www.eurocooling.com/articlerhodia.htm

Lots of good comparison graphs
its a/b compatiable ( synthetic or mineral )
operation pressures similliar, slight overall capacity decrease, eff increase even with longer run times
Ozone friendly, R-22 compressor

Do you know an approximate cost per pound for it?

that last post was wrong wayyyyy wong
still looking for a price in dallors