Well, since there's not a lot of info out there I decided to check for myself. I used my Keurig to brew up a couple of cups of "Truckstop coffee" using each of these malts run through a coffee grinder. Then I spiked an Amber Ale I had laying around that is actually pretty similar to my porter, minus the roasted malts.
I ended up using 30mL (about an ounce) of the coffee to 4 oz of the Amber. At that point, the color was still a bit lighter than a porter, but it had darkened significantly. The dark roast barley-spiked beer was a bit darker than the light roast, but only by maybe 1-2 SRM tops. My WAG is that it went from about 16 SRM initially to 20 and 22 SRM. Neither had any impact on head color - it remained cream/off-white in color.
The light roast barley beer had roasted notes that are similar to a medium roast coffee. The roast character had a bit of roughness, but it wasn't egregiously bad. The finish dropped off smoothly without any additional harshness. I didn't pick up any ashtray notes.
The dark roast barley beer had deeper roast notes than the light roast. It reminds me more of espresso. The roast is quite smooth and tends to "fade in" a bit as the finish goes on. There is also a bit more acidity here than with the light roast (maybe this explains why the roast is a bit smoother). The finish is longer than the light roast, but still seems smoother. I did pick up a hint of ash at the tail end, however.
I also tried a 50-50 blend of the two "malt coffees". What I ended up with was the harsher roast notes of the light roast with the ashiness of the dark roast at the end. Not what I was hoping for.
A quick caveat, the grains were ground pretty fine, so I'm sure that some made it into the beer. This likely explains some of the ashiness from the dark roast. I think I can find a use for each of these malts, but the dark roast was my favorite between the two. I also prefer dark roast coffee and *extremely* dark chocolate, so this may just be my personal taste.
This was my first time using the Keurig for something like this, and I'm pretty happy with how it turned out. I will definitely be doing a bit more experimentation with specialty grains using this method. I will see if I can get decent extraction with a coarser grind next time to keep husk particles to a minimum. And in case anyone was wondering, coffee comes out of my Keurig at 168F so this seems like it was meant for testing grains.