So you think you need more than gig? Or linking multiple 100mbit ports? Does this nas your looking to buy support that? You can move some serious speed across a gig connection. Unless you talking backup of ssd to ssd I don't think the gig wire is going to be your bottle neck.
Any nas you buy I would think would have gig, and gig switches are cheap.. And if you have to update nics in your other boxes, again cheap..
I see 70 to 90MBytes per second over gig without any issues.. And that is on CHEAP ass hardware to and from a VM
Would 90MBytes per second not be fast enough for your backups?