I just found out that the Archer C7 now has support for both Qualcomm SFE and hardware decryption. No official builds yet (all official builds for the C7 are based on kernel 4.9 while SFE requires 4.14), but I found this: https://github.com/gwlim/openwrt-sfe-flowoffload

For me the speedup with SFE is almost x2, haven't tried hardware encryption yet.

With this, I would say the C7 is an excellent router for OpenWRT.