Following up on this. . .
Been trying the X4S over the last 24 hours and there's no discernible improvement. One device marginally faster on 2.4, however, two Apple devices both noticeably slower. Range is really no different.
Hard to justify the purchase on that basis. It's going back.
What achieved an improvement was fully disconnecting the Asus (power supply and all). Then, later during testing, hooking it up again. I think the short rest did it good! Speed and range appear to have returned to where they've been in the past.
Anyway, doesn't solve the dead spot. So I'm thinking of getting a powerline with Wifi clone feature.