Hi,
We have a very similar problem where on a Dell XPS 13 and XPS 12 in conjunction with the SMSC LAN7500 adaptor. It works perfectly with the provided drivers in XP, Windows 7 and Windows 8. It PXE boots correctly as well but the drivers just will not work with WinPE 2.1. We use Altiris 6.9 SP4 for deployments and I have heard that Symantec have not even changed the support WinPE version in 7.1, perhaps even 7.5. I have spent ours messing around with "devcon", proving that the laptop can see the hardware just fine, it just won't accept the drivers.
If anyone has got this to work I would be most grateful.
Howard