So I've been testing some barbarian things. Recently, I was testing glancing blows with and without each tier of frenzied berserker as well as the barbarian might. Each video was about 10 minutes long, but I found that I got a bit more data from the video with barbarian capstone than the others. I haven't really been looking into attack speeds, because my setup isn't really built for it, but this looked to be worth looking into. The videos were recorded at 5 frames per second, although between 1/3 and 1/2 of the frames are just repeated frames. The same character was used, a level 20 human pure barbarian. The only difference between the tests were running to the barbarian trainer, getting another enhancement, then running back out to Ataraxia to whack on scrags. I was using a two-handed weapon. I did not twitch, but just did auto-attack. I didn't use any buffs like rage or haste.
For frenzied berserker 3 but without the barbarian capstone, it took a total of 623.6 seconds to get through 892 attacks. This translates to an attacks per minute of 85.8 seconds. The measurement error, assuming that there may have been a repeated frame at both ends (i.e. at both start and end), is 0.4 seconds each, which means a measurement error of about 0.6 seconds total. Out of over 600 seconds, this translates to less than 0.1% error (note: this doesn't mean that the attack rate is always like this, since your attacks per minute will depend on graphics settings, client loading, server traffic, etc., but just that it wasn't an issue here).
For frenzied berserker 3 with the barbarian capstone, it took a total of 614.0 seconds to get through 964 attacks. This translates to an attacks per minute of about 94.2. I didn't take any other enhancements between not having the capstone and getting the capstone.
This means that the barbarian capstone currently seems to give about a 9-10% attack speed increase (from 85.8 to 94.2 attacks per minute).
On suggestion from Shade, I recorded a video without and without the capstone. The video consists of me spending three minutes swinging at air next to the trainer, then getting the barbarian capstone enhancement, then swinging at air for another three minutes. It is one continuous video so there shouldn't be issues with (for example) changing the video settings in between tests or something. The video was captured at 20 frames per second, although at first glance it seems like this just means more repeated frames. You can see the video here:
http://www.youtube.com/watch?v=y_Yz8dWo5ag
In the video, without the capstone, it took a total of 183 seconds (from frame 81 to frame 3741) to do 260 attacks. This is 85.2 attacks per minute. With the capstone, it took a total of 187.1 seconds (from frame 4195 to 7937) to do 292 attacks, or 93.6 attacks per minute. The testing results for both were slightly lower (which may reflect increased graphics demand since I upped the video capture to 20 frames per second instead of 10 frames per second) but the ratio is still the same, about a 9-10% attack speed increase.
I do not know if this trend holds up for TWF or unarmed, but people who are considering barb with splash might want to consider this hidden effect that barbarian might seems to give. I also don't know how this may affect twitching, since I didn't bother trying to twitch because mine isn't very consistent and would just screw up any testing results.