Again, I think better armor = more wounds is an interesting idea, and will be good to put out there for Xth edition, but I think it would have to great a ripple affect on the current edition. For it to work, other codexes would need to be altered in kind.
Also, I'm assuming 3 wound TDA troops would be very expensive. With T4 and 3+ save (and no invulnerable save), there will be many, many things that can waste them (and a HUGE chunk of your points) with ease (ie, krak missiles, power fist, battle cannons).
I agree TDA should be a bit better than it is, it used to be very powerful, but I'm not sure if adding wounds is the way to go. Maybe a bonus to Toughness or Feel No Pain, maybe re-roll failed armor saves and a better invulnerable?