I’m considering purchasing the Eero Mesh system. My main concern is the speed I would be getting. I’ve tried an extender in the past. Returned it because it slowed my network. Will I see the same speeds I’m getting now (300-350mbps). 
I’d probably get the main unit with one “satellite” unit at the opposite end of the house.
And, will it switch from 2.4 to 5GHz automatically?