The time at DARC would be fun... but so would hanging out on the beach. So the question is what will benefit you the most in your progression to 18A? I got to do some work at T.E.E.S. before joining the Army. I promise you, every instructor/cadre could give a rat's-ass if you've been to some "high speed school" unless it's in the ATRRS listing. If anything it will put a bulls-eye on you as you'll be considered the guy who thinks he knows what he's talking about. There are plenty of "wanna-be's" that attend all sorts of good training.
At DARC you'll probably never be faced with something that may weed you out. You'll probably not PT every morning unless it's on your own. You'll not be required to do something that "really" sucks. There will be no gut-check. It's as hard or as easy as you want it to be. Rich (and schools such as his) are in the business to make money. It's a different world than most army schools.
That said- you show up at a unit with a Ranger tab - everyone knows there is some level of difficulty that you've taken on and passed. You'll learn things the "Army" way and that's what you'll need in IOBC, and many of the Phases of the Q-course. Sapper will give you more knowledge that is useful to the Q-Course than DARC. Airborne - at least you won't be a leg..
Again, the time at DARC would be fun but it will do little to nothing to help you one your way to 18A.