Ah, ok. Re-reading your original post, that makes sense--my fault for not getting it. Sorry about that.
I'm not 100% sure, but I think the power drop with voltage will be dependent on the amp design--some might be linear, and others less so. I would guess that Class A or AB amps would be more linear, and Class D amps less linear. IF your amp is linear, it's going to be good for about 70W at 13.5V, and 73W at 14V. Not a huge difference, but...
From a practical perspective, you don't really have anything to worry about. There's likely a bit of a safety margin in the Focals' power rating, the amp's rating is going to be on the optimistic side, and there's going to be some reduction in power due to the voltage, even if we don't know exactly how much. Also, when playing actual music, you're never going to get 100% of the amp's rated power delivered to the speakers. You might be able to pull it off with pure sine waves or white noise or something.