Does the new Airport Utility also make new FW available to the router?
If it was the update that was posted on July 11th (6.3.1), then no, unfortunately.
I purchased the Extreme a bit over two weeks ago and did quite a bit of non-professional testing (speedtest.net, or moving files around the network at different points throughout my home) and came to realization that my particular model was defective.
It was terribly inconsistent, regardless of the restores/reboots or devices I used. Apparently, this was occurring to some individuals, so I returned the day after and replaced it with a Time Capsule seeing as they no longer had any Extremes left to exchange.
Didn't care much for the hard drive, but eh, over the air back-ups sounded convenient, if not overpriced imho.
I did comparison tests against my current/previous router I was using, the Asus N66U. Great router, but I always had great experiences with AirPort models, so I bit the bullet for fun to check this new one out. I unfortunately have no 802.11AC based devices, so my experience was purely based off 802.11N on a combination of 2.4/5Ghz ranges.
Performance was initially poorer then I was hoping for, but it was certainly working normally. Speeds were consistent and network transfer never failed (unlike the prior Extreme) and were also pretty consistent. I was achieving roughly 28MB/s on average between a wired Windows 8 Desktop to a 2010 MacBook Pro 13" (OSX 10.8.4) on the 5Ghz band within three feet of the router. Faster by a few Megabytes then my ASUS.
For the life of me, I can't figure out why the router was doing so poorly in certain locations where the Asus would trump it. I wasn't expecting the TC to beat the ASUS, but at least stay close, particularly with mobiles devices on the network.
I used a pair of iPhone 5's which I would switch between 2.4 & 5Ghz Bands as well as a 4S on the 2.4Ghz Band, among other devices. Speeds just didn't make sense.
Locations where I could easily hit 70 Mbit/s on 5Ghz would crawl to 35/22 range. They were consistent though, so I figured that was just the unfortunate extent of I what I would expect.
I decided to restore the TC to factory settings, just quickly set it up on iOS and speeds were far superior, stellar actually, beating the ASUS in both range and file transfer speeds in the same locations. But after awhile, they would revert back to their poor speeds. Rebooting the router did nothing to solve the problem. A second restore, once again brought speeds and range back up. I figured it must have been a client I was connecting the router to, possibly being finicky with the router, but, no dice after some more restores tests.
I'm going to continue to do odd tests with it, in hopes it might just be a firmware fix, otherwise, I certainly use this as my full time router. It's almost as if the router is dropping output power after a certain point and can be resolved (temporarily) through a restore.