They definately are.
I was 8 kg's over on the domestic portion of my trip to the UK, and for the domestic flight they wanted to slug me $80. They even wrote out the paperwork that I had to take to ticketing. As well, one of my bags was over 32kg, so that wouldn't be accepted. Staff member was very apologetic, but she couldn't budge.
Removal of excess stuff and shuffling got my total to 38.3kg (31.8kg on the heavy bag), which was accepted (38Kg as a WP).
In Sydney, I checked in at the F counter and no mention of weights was made (bags didn't really change weight). Same in Hong Kong when I checked in.
So don't know what flexibility the F check-in staff have.
(PS, the only reason I was overweight was the hotel I was staying in didn't have scales, so couldn't check before heading to the airport).