Clocks don't tell the whole story. Possible a 2100Mhz custom card = a 2150MHz FE card in benchmarks.
I do hear a lot that FE cards clock pretty good, considering the weaker pcb and single 8pin connector.
Another thing Nvidia possibly did with the FE card's pcd was add or tune some chip( eg, PWM frequency?), and custom card makers don't know about it. Just speculating, I don't know or have the tech knowledge about it.
Just glad my custom card benches around 2150MHz with high mem clocks, ofcourse it's on water.