I welcome a correction, but my understanding is that the X540 are needlessly toasty - even by 10Gbe NIC standards. Maybe take aim for the X550 instead?
Also, if you go SFP+ ; use DAC where ever you can. They produce almost no heat since there's no conversion of the signal.
Mellanox connectx-3's are wonderfull. Mine may be a counterfeit, I have no idea, but it works great and runs cool. Only downside is the cards limited support for sleep. The card is low power in it self, but it will keep the cpu from going in to lower levels of rest during inactivety. So keep that in mind if you plan to use it for something that idles a lot.
What's the goal? Purge gigabit, or keep some connections? What connections and why? How many machines, and what kind och PCIe slots ( lanes and version) do you intendent to use?
Good luck!