Poor Time Capsule Performance in a Roaming Configuration

Question

I have a bit of an odd problem.  I recently added a Time Capsule (4th Gen) to my network alongside my existing Airport Extreme (4th Gen).  Both devices are set in Bridge Mode and connected to a 1000Base-T backbone which accesses the Internet through my AT&T U-Verse gateway (WiFi off). My intention is to have the devices in a roaming network with the AE upstairs and the TC downstairs in my office. 
Coverage-wise this has been excellent, but performance has not been up to par.  Through a great deal of troubleshooting, hair-pulling, and even a replacement TC, I have determined that, for some reason, the TC has trouble forwarding my frames if it is on the same SSID as the AE.  (Which is, of course, a required for a real roaming network.)
Both devices are running 7.6 firmware and I have manually set the radio channels at the far extremes from each other in both bands to rule out interference.   My neighbors' APs are still visible, but aren't causing any intereference in those bands.
If I configure both devices with identical WiFi settings (SSID, WPA2-Personal, PSK, etc.) devices that are associated with the TC experience very poor performance. The radio part of the connection is great.  (Strong signal, low noise, etc.)  Data throughput is awful, though.  Web pages won't load, and when I ping local devices on the wired side of the network I'm lucky to get 500ms round-trip. Most of the pings are in the >1000ms range with many timeouts. 
Devices connected to the AE, however, average about 1ms round-trip to the same devies and see no issues getting to the Internet or transfering files locally.  In fact, Time Machine backups work just fine to the TC.
That said, if I simply change the SSID for the TC radio, leaving *everything else* the same, it works like a dream. Unfortunately, that isn't exactly the roaming network I would like to have.
Has anybody else seen this behavior or have any ideas on how to fix it?  I'm stumped.  What really concerns me is that I've seen this with two different TCs.
For the record, I have already reflashed, defaulted, and configured both devices from scratch with no change in performance.

Answer 1

Replying to my own post is probably bad juju, but I thought I'd provide an update for those who may have also seen this issue.
First, it turns out that having the separate SSIDs didn't actually matter.  WIthin a day or two of my OP, the Time Capsule on the separate SSID began exhibiting the same problem as before.  It seems that this is part of the problem.  When it occurs it can take a few days to a week to turn up, but it can also happen right away.
Eventually, the Time Capsule began to exhibit other issues as well.  For instance, in my testing, I would need to reboot it to apply new settings, but it would hang on a reboot for 20 minutes or more and I would have to pull the power cable. I finally called Apple and they replaced it again.
When the new Time Capsule arrived, everything seemed to work well, but before I was even able to get the backups moved from the old one to the new one, the problem showed up on the new one. 
At this point, I found a few forum threads where other people were having similar problems and were able to get around them by downgrading form firmware 7.6 to 7.5.2.  I was about to try that when Apple released 7.6.1.
On a whim, I decided to try the new release, even though Apple only referred to vague WiFi performance fixes.  I have to say that I noticed immediate improvement across the board.
So far, so good.  I am hopeful that the new 7.6.1 is the fix to the problem.