In the specific case of the condensing unit, it will only switch into full wave mode at lower speeds, where the load is a fraction of full speed.

And relay requires holding current and it's an added cost and complexity.

Does switching to two diode from bridge at low load reduce load as measured at primary side of distribution transformer and is it to an extent justifiable to equipment owner to pull a 3 wire rather than 2 wire circuit?

You will have to deduct any power consumed by the relay and controls associated with it from reduction, obviously. . .

If the US can get IEC like regulations they have in Europe and includes harmonic content regulations on things intended to plug-in, I find supply side enforcement to be the most effective just like they already do for things that do not meet energy standards. It intercepts and sends back home incoming foreign shipments as well as imposing fines on companies who sell non conforming goods.

http://energy.gov/gc/enforcement-news

As many items are generally produced overseas, these non-compliant goods are generally kicked back out home by US Customs & Border Protection.

http://www.hlregulation.com/2013/07/24/cbp-publishes/