Anyone who has worked with air conditioning knows that R410a operates at higher pressures than R22 or R290 and therefore requires a complete redesign of the system.
For example, at 115F condensing temperature, R410a operates at a high side pressure of 390PSI.
In contrast, R290 operates at only 214PSI at the same condensing temperature.
Here's an easy way to visualize the pressures: 390PSI is the weight of "fat" Nina Davuluri concentrated over the area of a penny.
And 214PSI is the weight of "skinny" Nina Davuluri concentrated over the area of the same penny.
Nina Davuluri Was Plus-Size Before Winning Miss America, And Calling Mallory Hagan (That kind of makes R410a look like a step backwards, doesn't it?)
It should be obvious that attempting to use R410a in a compressor designed for R22 (or R290) would result in catastrophic failure. But what about the reverse? The compressor would then be "overbuilt" for the refrigerant. With the "Davuluri Treatment", a cheap window A/C can be hacked into a heat pump water heater (that is able to heat the water hot enough for effective dishwashing) or heat pump clothes dryer. It is still necessary to keep the discharge temperature under control, but at least going to higher condensing temperatures wouldn't result in insane pressures. (Just look up the pressure R410a would operate at in a 140F water heater!)
Under more usual operating conditions, I have noted that the compressor I'm using ran significantly quieter. It also seemed to dim the lights less when it starts, even if the start capacitor circuit (optional on rotary compressors) was disconnected. There is, of course, a capacity loss due to the lower displacement, but the power usage also goes down so overall efficiency would be about the same or slightly better. (That could actually be a real blessing if you have a really oversized unit. Recover the R410a, swap out the TXV and filter drier, fill it up with R290, and slow down the evaporator fan.) I remember reading about one or two others who have converted a R410a system to R290 and more or less got the same result I did.
When dehumidification is not needed, there's yet another trick to boost efficiency. Simply run the evaporator fan(s) up high to get the evaporator temperature up and thus the EER. The high suction pressure could overload a regular compressor, but the R410a compressor running on R290 would still be well within the suction pressure limit.