I honestly don't think any router can do multiple Xbox One's with Instant-On. Seems like a deisgn flaw on Microsoft's part and needs a system update. I haven't seen a situation where my non priority devices on the DGL-5500 didn't get bandwidth when other devices were offline. My PC is 3rd on priority after the xbox's and it gets pretty much full bandwidth on speedtests.
I had tested the Linksys EA6500 which has a similar priority function, this was my rough testing speeds in various situations;
Ping Network Traffic Internet Speed Test
Priority Off 19-25ms Low 101/34mbs
Priority Off 100-158ms High 101/34mbs
Priority On 19-28ms Low 38/8mbs
Priority On 30-50ms High 38/8mbs
So as you can see it helps keep low ping rates in games but the bandwidth of other computers is always cut low.