Table data taken from Texas Leaguers for the 2011 season. Four-seamer and two-seamer got combined since I don't actually think he has a two-seamer. Slider got renamed for better illustration of the pitch's lack of quality.
Danks' fastball is average-ish. It's a four-seamer with a nice amount of rise but somewhat more than ideal run. The whiff rate against both RHB and LHB suggests it's below average, as 19% and 15% respectively were average for all lefty fastballs in 2010 (relievers included). But we're dealing with some fairly crude metrics here, with similarly rough averages to compare them to. What we know about typical fastball velocity and movement says Danks is right in that nice clump of average-ishness and I'm willing to round up having watched him for so long.
We must also consider another mitigating factor where his heater whiff rate is concerned. Namely: the cutter. Most lefties don't have two out pitches for their encounters with RHB, which I would be willing to guess puts Danks' fastball at the mercy of selection bias. That is, he trades off the possibility of a whiff upstairs on the fastball for a cutter up and in. There's still plenty of chance for a swing and a miss, but if the whiff isn't there Danks can still get a weak grounder out of the exchange. Four-seamers lead to fly balls and fly balls lead to home runs. Better to be safe than sorry in two-strike counts, no?
So perhaps that's it. Or perhaps there's something beyond the actual movement and velocity that's causing the pitch to suffer in performance. I'll give him the benefit of the doubt.
If we trace the movement of Danks' change on Dave Allen's heat map we see it's in that nice green blob suggesting a solid pitch. Not so much movement that he achieves dominance, not so little that it gets raked:
That's not quite a fair analysis, as I'd rate it a plus pitch on the whole. Its whiff rate against RHB is 15% above all lefty changes thrown in 2010. For some weird reason, it's almost 40% worse against LHB. I'm not sure exactly what explains the gap, but at least some of it is sample size. Since 2009, Danks has thrown more than 1400 to righties and just over 200 to lefties. Even after weighting the results, the pitch comes out above average compared to all changes thrown to all batters.
If I had to guess what's guiding the numbers there, it's the fact that it's very much a "feel" pitch for him. Some days, he's on the mound and in absolute control, putting it just on or just off that outside corner at will. Others, it's just not there. He'll leave it up, leave it out and often he'll end up visibly frustrated with it. And but so with only 200 pitches, it doesn't take that many bad luck outings to really screw with the numbers. He's incredibly entertaining to watch when he's feeling it as he'll absolutely breeze through lineups. It's also worth mentioning that I think his disguise of the pitch is excellent. Even when the command or movement isn't there, he gives very little if anything away in the delivery.
Ah, the piece de resistance. I don't have any averages to compare this to, but it's clearly a plus pitch. The real key is just how hard he throws it. On average, it's faster than Buehrle's four-seamer and a decent number of other notable lefties for that matter. Picking it out from his fastball is nigh impossible as it's just a smidge slower and ends up a full 6 inches farther inside than the expected four-seamer. For reference, that's more than a third of the plate. That far inside is almost always off the barrel and often enough it's off the bat entirely. Ground balls and whiffs, what more could you want?
He's also got a slider thrown at curveball velocity. It's not a good pitch. Definition of show-me. Fortunately, he's got a good feel for when to throw it and the results have been pretty good. Let's hope it stays that way.
Sum Of The Parts
Danks is very well equipped to attack RHB and for his career he's basically got no platoon splits. So far he's had enough velocity and been able to rely on that plus the cutter to get LHB out. I would guess he'd like to have a more legitimate out pitch to throw to lefties, but he's battled successfully to this point.
His command every so often will get in the way of his stuff. In particular, as noted, he'll lose the feel for his change-up and its location and movement suffer. It's not a huge problem, as his cutter is a very versatile pitch that can be thrown effectively to either side of the plate if necessary. But I wonder if learning the cutter at all didn't to some degree hinder his mastery of the change. It's the rare pitcher who can consistently move the ball in and out regardless of pitch and I do not believe Danks is an exception. At some point, he chose the cutter as his bread and butter, likely with Coop's guidance, and it's earned him a very nice payday. But every minute practicing the cutter is a minute not practicing something else. I say that not as a criticism--Coop and Danks are a terrific match--but to temper fan expectations. It would be great if he could put it all together and become a Cy Young type. That's likely not in the cards.