Like Gary said. Most Toto's leave about 3/4" behind the tank on a 12" rough in. (There are a couple that use the whole space, but they are prominently-labelled as such, because it's unusual.) The Aquia 2-piece (CST416M) spec sheet shows 1/2" from the back of the toilet to the wall, so if it is truly 11.5" from the finished wall to the centerline of the closet flange, the Aquia should still just fit, but it might be tight because of differences in casting the toilet. There are ways to "cheat" the toilet adapter (or toilet) a bit forward on the flange, because the outflow hole is only 2.125, and the flange is at least 3", so you get 3/8" or so that you can slide it forward and still have it dump into the hole in the floor. But this can be a bit tricky so come back for more info if you have a problem. Careful about whether you have any molding around the base though, because that will start to be a problem with the Aquia.
The CST 412MF (which is the Universal Height version of the 416M) spec sheet now shows 3/4" from tank to wall, so that should fit, although again there may be some variances in casting.
The Aquia 1-piece (MS654114MF) spec sheet no longer shows a tank-to-wall measurement, so I would beware, although I think 1/2" is likely.
The Original Toto Drake (model CST744E or CST744EN and the like) can happily mount on just under 11 inches (i.e. 10-15/16" or so), so that's also one to look at. Indeed, that very-very popular toilet might even look a little better on 11.5 than it does on 12, because there is less space behind the tank to the wall on 11.5. Be sure to check out the look of the new tank on the "EN" version, because a lot of people seem to like it.
So now let's talk about skid marks. (Do we have to? I guess so...) Here's my take on it: People need something to bitch about, and you see a lot of that online. The Aquia is a washdown flush, which means that there is no siphon jet at the bottom of the bowl to power the flush. That siphon jet is a common feature in toilets, and it helps, but it also uses water -- water goes out of the tank and down the drain but never passes across the surface of the bowl. In a washdown flush, you get a big cascade of water -- all of the water used in the flush -- across the surface of the toilet bowl. This helps dislodge skid marks. Washdown flush toilets generally hold less water in bowl upon "refill" than do more-conventional toilets, so stuff has a greater chance of touching porcelain without first hitting water, which sometimes makes a difference in whether it attaches to the surface and needs to be dislodged (but, I'm here to tell you, even in 9-gallon toilets, sometimes it doesn't). So, on balance, you may have a little more of your business adhering to the Aquia before the flush, but then the flush is a pretty-darn-dramatic cascade of water (for a low-flow), so there's a greater chance of it cleaning off. It also depends, as is often pointed out here, on the (God help us) "consistency" of the stool.
Here's what I have found: a lot of people who are installing low-flows are replacing favorite old toilets where they poop into a swimming-pool-sized body of water, which, when flushed, lazily swirls its way around in the bowl before finally discharging 20 seconds later. When they start using a new toilet that doesn't put 2 gallons of water in the bowl to poop in, they find that OMG it's different, and some look to find fault, while others are amazed at how much can be done with 1.28 (or even 1.0) gpf!
That said, I have on rare occasion walked into a bathroom in our country home and discovered that the previous user had made a particularly-sticky deposit, and there was a skid mark or two sitting in the water of the bowl. You know what I do? I flush. After having sat there for hours, because of the solvent action of the water, it just comes right off and goes down and voila!, clean again. This happens about once every 100 visits.
The Aquia flushes very well, in terms of evacuating the waste in the bowl, and it does a good job of cleaning. You may find instructive the experience that our founder, Terry, relates. He has installed literally hundreds and hundreds of Aquias (and thousands of Totos) in his market. He says that the Aquias have the lowest return rate (less then 1/2 of 1 percent) of any toilets he installs. And, on top of that, he gets many, many, many customers who install one Aquia, and then call a month later and order more. That experience is pretty confidence-building to me. He also has very good experience with the Original Drake and the Drake II, as well as other Totos.
