If you produce [salary]$ worth of value, then it does not really matter how you produce it, right?
I'm not GP, and I'm self-employed, but I think it's fair to compensate people based on the value they create, and not how much time they spend at the office. If a guy gets his work done in half the time, and his employer is happy with the cost, then everyone gets what they want.
If you think they would react negatively, then wouldn’t you concede that there is an element of dishonesty here?