After reading contradictory answers, I decided to make some tests to determine what the correct way to add (2 x 8 GB) RAM to existing (2 x 4 GB) RAM. And this are the results I found.
Tested on iMac (27-inch Mid 2011) Intel Core i7 - 3.4 Ghz with High Sierra:
* With the following configuration from System Information / Memory (by BANK): ***
'''BANK 0'''/DIMM0: '''4 GB''' - DDR3 - 1333 Mhz
BANK 1/DIMM0: 8 GB - DDR3 - 1333 Mhz
'''BANK 0'''/DIMM1: '''4 GB''' - DDR3 - 1333 Mhz
BANK 1/DIMM1: 8 GB - DDR3 - 1333 Mhz
Geekbench 4 shows and memory bandwitdh at '''8.76 GB/sec''' for '''Single-Core''' and '''8.61 GB/sec''' for '''Multi-Core'''
(Full results at : https://browser.geekbench.com/v4/cpu/10127634)
* With the following configuration System Information / Memory (by DIMM): ***
BANK 0/'''DIMM0''': '''4 GB''' - DDR3 - 1333 Mhz
BANK 1'''/DIMM0''': '''4 GB''' - DDR3 - 1333 Mhz
BANK 0/DIMM1: 8 GB - DDR3 - 1333 Mhz
BANK 1/DIMM1: 8 GB - DDR3 - 1333 Mhz
Geekbench 4 shows and memory bandwitdh at '''15.4 GB/sec''' for '''Single-Core''' and '''16.6 GB/sec''' for '''Multi-Core'''
(Full results at : https://browser.geekbench.com/v4/cpu/10127466)
The good configuration to exploit the Dual Channel to get the best performance is '''“by DIMM”''' meaning to have same RAM sizes in same DIMM number.
Daniel