Hey Captain, ultimately it is a matter of preference. You can get the same job done either way. High efficiency cost more up front for more COBs but you need less heatsink/drivers etc. Lower efficiency requires less COBs, but more dissipation watts/heatsinks/drivers/electricity/cooling/ventilation to get the same nugs.
I suspect those eBay drivers will run softer once they are warmed up, 1300-1400mA. At 1400mA the CXA3070 3000K Z4 is 39.3% efficient and dissipates 52W (20.4 PAR Watts = $3.33/PAR Watt all in cost). That is pretty tremendous and considering how small of an area it covers when you keep them in tight to the canopy that is a lot of photons in the canopy.
If you bump it up to 1.8A you get 36% and 69.7W (25 PAR Watts) which is still really awesome performance, but it doesnt save enough money to tempt me. It may not be possible to maintain a Tj of 50C so those numbers could be lower. Also, I can not give you an all in cost because I am not aware of any cheap drivers that can do the job.
I decided to run them at 900mA which is 43.7% efficient, dissipates 32W (14 PAR Watts) and cost $4.64/PAR W all in cost. There are some growers that are efficiency crazy and could run Z4s at 650mA with the $6 drivers. That would be 46.8% efficient, dissipate 22.6W (10.6 PAR W) and cost $5.75/PAR Watt all in cost. So that is 56% more efficient than commercial panels and still cost less.
For comparison, the Onyx Grow is probably the best commercial grow light. It dissipates 210W at 30% so that is about 63 PAR Watts. It cost $450 so that is $7.14/PAR Watt all in cost. These numbers are directly comparable because the Onyx grow uses 3000K Cree except the Onyx will suffer a significant additional penalty due to lens losses.