One more datapoint that's valid only if you're *A gold:
If you're in SKG you board directly from the lounge for A3 flights. I've never ever ever ever seen a gate agent check customer's bag sizes there(=in the lounge) in all my flights in the last 10+ years . Some flyers even board with carry-on+ laptop + 2 duty free bags + a terkenlis "tsoureki" and noone seems to care

In ATH, I'm usually in the lounge until its about boarding time. Then, when I go to the gate I sit as far away as possible. When priority boarding starts I just get up and walk the 20m to the gate along with other priority boarding customers. I dont think gate agents check the luggage size that much, if at all, at this point

In other words : I've seen gate agents weighing/measuring priority passengers who show up last minute at the gate.