In case anyone else is interested in an extremely nerdy and unofficial look at the numbers:
Using timestamps of posts in this thread (and from a couple other "interested" forums) that mention specific counts, I found ~60 data points from early on the 5th through about noon today, plus a handful of my own this evening. Comparing the gaps of time and counts between pairs of posts, I can work out the approximate signups/hour over various periods of time. Here's the quick rundown:
From the "start" (counting the 8/5 early-morning announcement) to now: 14.2 signups / hour
Since the "race to 10k" announcement, approximately (5pm GMT yesterday, if my calc from EST is right, with the count at 7075) to now: 19.3 signups / hour
Needed from now (04:00 GMT) to the end of the 31st: 24.6 signups / hour
It's a bump in the average, for sure -- and we've already seen a bit of bump since the extension announcement. But the rate has been > 25 / hour for several stretches during the GB, including a full day near the beginning and at least a 10-hour stretch today.
This is my first IKGB rodeo, so I'll defer to others with experience about how high, far, wide and long the numbers might or might not jump, but 10k seems plausible if enthusiasm holds. In the meantime, the numbers are fun for a data nerd to fiddle with.