Well, first of all, one minor point: the fact that you can use a modified rotation and hit Impulse Drive before the previous Disembowel wears off doesn't mean that you've reached the break point for a DPS gain. What I really did was I just assumed (correctly) that, at very high Skill Speeds, ID clipping is something that will probably happen. Thus, I built my equation around that assumption, which helped me to more accurately calculate the 2.12-second cut-off.
Second of all... actually... a while back, I found an error in the equation as it's written on the main page here (bad Thendiel), and determined that a more accurate break point value would be about 2.06s. Just for the record. I'll fix my write-up sometime soon.
In any case, you're right that Arrow definitely has the potential to be awesome, but it also has the potential to place us in the "dead zone" of sub-optimal Geirskogul use that I've talked about, particularly if one receives Arrow late in a one-minute BotD cycle. It may well be that the DPS gain from the Skill Speed boost is worth that trade-off, but it *would* be a trade-off. On the other hand, if you get Arrow early in a BotD cycle and can crank out extra Geirskoguls per minute, then, yeah, it's purely fantastic.
It's probably best just to keep your item level as high as possible, since the benefits you get in terms of strength and secondary stats likely outweigh the disadvantages of sub-optimal Geirskogul use. It's not that a GCD length of 2.31s to 2.38s is apocalyptically bad; it's just bad in a way that isn't reflected by the stat weights, so, given reasonable gear flexibility, we'd want to shoot for sub-660 SS. Here is a write-up that I made which illustrates the disadvantages of 2.31s-2.38s. In it, I peg the associated DPS decrease at about 0.7%.
In the meantime, try toying around with Ariyala's like Jack recommended, and keep an eye out for opportunities to lower your Skill Speed with future Gordian Manifesto Page purchases.