Just hit 60 DRG and played around with it for about an hour. Here are a my initial thoughts/observations.
First, they must've really toned down the GCD reductions granted by Skill Speed per point. I'm at 592 SS and my GCD is still a whopping 2.41. That 2.30 second threshold I was theorizing about for Geirskogul use? Not gonna happen. I would guess that that change was intended to lighten the TP decimation that used to come with high Skill Speed. One assumes that it's offset by the SS benefits to DoT damage.
Second, the threshold of BotD duration for popping Geirskogul when BotD is recovering from cooldown is about 22 seconds for me--close to my prediction.
Third, I'm finding that Geirskogul use is pretty consistent for single-target Striking Dummy conditions. Basically, you want to go HT - combo - Geir - Phleb - combo - HT - combo - Geir - Phleb - combo - HT - combo - geir - Phleb - ID - Dis [BotD falls off and needs to be immediately refreshed] - CT - 4 - Geir - HT
...and then something very similar repeats.
If all that sounds confusing, I promise it has a relatively intuitive feel to it with some practice. Basically, every oddly-numbered combo should be followed up by Geirskogul, except after BotD comes off cooldown, when it switches to evens. Combos number 1, 3, and 5; 6, 8, and 10; 11, 13, and 15; 16... are all followed up with a Geirskogul. Also, the timing feels really tight here at my current Skill Speed, so there's probably a nearby GCD length above which this approach won't work. 2.43ish maybe?
Fourthly, based on the short cooldown of BotD and the bosses I've fought in dungeons so far, my gut tells me that we're going to have a ton of downtime in endgame fights, forcing us to think more creatively and responsively about how we play. In other words, this optimum Geirskogul stuff will probably only apply to one or two fights that matter.
Finally, I'll just say that despite all the salt concerning the 4th combo procs, I am really, really, really happy with and excited about DRG at level 60. They've managed to inject several new mechanics into the job that require care and attention to master, and it all feels very fresh and interesting. I can just... feel the potential for insane damage in endgame. My take? SE absolutely nailed it.