No solutions, but pehaps some useful info:
After isolating the problem to my retina 15" MBP, with documented tests showing the erratic ping vs. older MBs on the same network with no erratic ping, I felt confident going to the Apple store to demonstrate a problem.
While waiting for my appointment, I decided to test several of the MBPr on display, which all demonstrated the same erratic ping (1-->200ms)! I then testing one of the older non-retina MBP on display, and it didn't have this issue. I was then pretty deflated when I got to my appointment, as it was clear ALL the MBPr were exhibiting the same issue. The Apple techs could not explain this, nor had they heard of this issue. After lots of testing, they sent me on my way with a clean bill of health.
I have since played around with the pings, and found something very interesting when I modified the ping settings. when I sped up the pings to go every tenth of a second, the erratic ping mostly went away:
544 packets transmitted, 544 packets received, 0.0% packet loss (0.1s ping 802.11n)
round-trip min/avg/max/stddev = 1.352/3.022/131.925/5.956 ms
534 packets transmitted, 534 packets received, 0.0% packet loss (0.1s ping 802.11ac)
round-trip min/avg/max/stddev = 1.452/2.725/103.777/4.430 ms
56 packets transmitted, 56 packets received, 0.0% packet loss (1s ping 802.11n)
round-trip min/avg/max/stddev = 1.548/131.013/301.689/97.062 ms
55 packets transmitted, 55 packets received, 0.0% packet loss (1s ping 802.11ac)
round-trip min/avg/max/stddev = 1.569/80.722/1000.725/148.826 ms
Does this make sense to anyone?