For most cases you shouldn't need iPXE with UEFI. The bhyve UEFI image has a built-in driver for virtio-net, and UEFI itself supports boot via PXE.
However, the e1000 does not have a built-in UEFI driver, and iPXE does some support some arcane network-boot protocols that UEFI doesn't. In addition, having support for option ROMs in bhyve is required for graphics-adapter pass-thru.
It isn't too difficult to implement, and will show up at some point.