So an update on the Guardian Tank now that I started Chapter 3 with him. Earlier concerns seem unfounded, I think. The Guardian Tank doesn't feel much more difficult than other classes. Obviously, being overleved does help quite a bit, which gives you the opportunity to just use Kira (DPS) along the way, so you go through battles quite quickly. I have not noticed, however, any significant increase in difficulty when fighting mobs around the same level as yourself (which I did when I went through the Alderaan Bonus Series), even with Kira as a companion. The only difference is that when fighting Elites, you better take Doc (Healer) with you.
Fighting one Elite is quite easy, even one at your level. You pretty much never get under 80% of your health, as long as you manage to interrupt some of their hardhitting skills. Fighting Elites with backup is a bit more challenging, but eventually, it's just a matter of taking out the backup quickly (while holding aggro so Doc isn't overwhelmed). When you do, the Elite's pretty much busted.
Some time around Voss is when the mobs are getting closer to your level again, so if it proves to be more of a challenge than I expected, I'll let everyone know.