Yes you can go all termie and of course they're better than Deathwing, they have a newer codex. GK are all about quality over quantity, in most cases at least (there's an inquisitor who can make your henchmen warbands into troops). Although I think that if you go all termie you'll see the same problems as with Deathwing; you lose 2-3 and sometimes that'll cost you the game. GK are expensive points wise, but they can easily be made tough as nails too. Everybody has nice armor, good gun, good melee weapons (your normal GK, not necessarily henchmen, etc), even a huge amount of psykers, nice vehicles. The best of everything, you just have to pay for it.
They're certainly not like other armies, like Orks, where I think you lose if your opponent doesn't kill enough of your Orks before the game ends.
Do go knocking Deathwing too hard, I'm sure they have a codex just around the corner and are about to become really viable once more.