In my experiences with both EMG's and Blackouts, here is what I've found...
*EMG's do have a notice-able difference between 9 and 18V. And it is also true that 27V really don't do anything. The ONLY time a 27V mod would be ideal/practical would be for certain EMG accessories, and even at that pretty much ALL of them benefit from an 18V mod. Like the pickups themselves, 27V mod on accessories usually warrants no notice-able differences.
*Blackouts do not benefit on anything other that a standard 9V system, as per how the preamp was designed. Unless you have a specialty Custom Shop active (the very limited "sevenstring.org" batch...) like someone I know, then trying any kind of voltage mod is a waste of time. Now I CAN tell you that having played the "Sevenstring.org" Custom Shop actives, those particular pickups CAN and DO benefit from an 18V mod, as well as running a preamp boost with it. The member who has these pickups has them installed in his guitar with a 9-18V switch, and an EMG PA-2. The PA-2 adds the extra 6dB of boost that was tweaked from the preamp initially - making it equivalent to an EMG X-series pickup. The 9-18V switch does exactly that, and runs the entire system at either 9 or 18V. So in this case not only do the pickups get an increase in headroom and dynamics, but also the PA-2 gets an extra "umph" to it, which in turn pushes the pickups a little harder. The owner loves them better than any other pickups he's owned or played, and he finds them to be "the perfect pickups" for playing stuff from Deftones "Around The Fur" album.
And yes while Duncan will make you anything you want as far as a pickup goes, are you prepared for the price tag that will come with it? For example: I had Derek give me a quote on having a coil split-able Blackout made - basically an EMG 707TW, but in a Blackout. He told me it could be done, but I was looking at $450 per pickup... Now while I would love to get, have and try something like this, I can not justify spending $900 for a set of pickups, regardless of how good they might be. Now if SD comes out with this in a new production model for the same price as an EMG 707TW, I'd be the first one to order them.
In terms of wanting the hottest set of pickups out there, I'd say either something like the LiveWires Metal or Blackouts Metal for active, or consider something like the Black Winters, X2N, Invader, Distortion, Dimebucker, BKP Aftermaths or anything along this line. I honestly don't see the need or point in trying to run actives at anything more than 18V for being useful and still getting a good tone from them, not to mention taking more wood out of the body and reducing tone that way to compensate for trying to run the extra voltage that won't give you any results anyhow. Crazy tone can be achieved and obtained via the amp, EQ's and specialty wirings on pickups depending on what your after. If your looking for over-the top, this thing goes WAY past 11 distortion and gain, I'd say you should consider looking at the amp, pedals and everything else other than pickups.