I don't think you quite understood what I meant by Cat 6 is utterly useless. Cat6 won't do any future proofing, it's a waste of money. All current and future speeds that will run on Cat 6 also runs fine on Cat 5.
You can easily run 1000mbps over Cat5, you don't even need Cat5e for it. There's no point to Cat 6 at all, as all future standards (i.e. 10gb+) require Cat6
a or above.
As for the electrical costs of running a switch, you're probably looking at about £5 per year or less, not the amount I'd worry about
