We faced the exact same issue with the new Lenovo models. I actually think we have have used the Windows 7 32-bit version of those drivers, since we are using the 32-bit Windows PE 5.0 with 9.6 SP1. (Which is actually based on Windows 8.1?)
In the Distribution > Operating system provisioning > Preboot > Manage Drivers in WinPE Image:
We found one of the Yoga/T450 drivers seemed to work well for both the T450, Yoga, and X1 Carbon models we just received.
I _think_ the one we have attached there is e1s6232.inf from TP_T450-T450s-SWG_W732\Ethernet\J5R204WW\WinPE (from the SCCM package on Lenovo's site)
