Alden, it appears that this is not a "Jacob's brake" as fitted to 18 wheelers that you hear noisily decellerating on entering cities. This is a standard exhaust brake actuated by a butterfly valve, rather than a system which adjusts timing. The critical point when selecting one of these is the controller for the torque lock-up. The exhaust brake will have very little effect unless the TC is locked. There are several manufacturers, of course, and I chose BD because of the controller. I have been using it for 3 years, and I wouldn't be without it. It does have limitations. The link you supplied gave no details of when the controller locks the TC. You need to find out if there is lock-up in all gears, or just some of the gears. You need to establish when lock-up is cancelled. e.g when the transmission is manually put into or out of overdrive. These factors are crucial when you are towing heavy, particularly on a down-grade. This is not the time to discover that the exhaust brake controller does not support 2nd gear lock-up. You probably know all this, but if you want more information, and you haven't been there, you'll get opinions about the Jacob's system at
www.turbodieselregister.com.
Nick