I'd rather not get into the whole mass driver asteroid debate. I don't think it's that feasible. Did you ever read the "Asteroids aren't cheap" article (I believe in WD) where they explained why the Imperium don't use Asteroids for Exterminatus. It's true the Tau may have better mass driver technology, and they wouldn't spend several weeks blessing the asteroid. But overall I don't think it's as straight forward as you make out.
Nastromo was very much a special case. Most planets don't have an adamantium crust, and heating up the core of a planet, which is already probably hotter than a lance shot anyway, wouldn't cause the planet to explode. Nastromo's crust was cracked from when the Night Lord's pod crashed into the planet (which is what the 8th Legion aimed at), and for some reason the planets core was explosive (why did anyone ever choose to colonize and explosive planet?). On any other planet, conventional weapons would have to kill the planet one blast radius at a time, expending ridiculous amounts of ammo, and giving the occupants of the planet time to escape the destruction. It would be mass-murder but not exterminatus.
If improving the Arco cannon was simply a matter of wanting it to be better, why doesn't someone just take a normal ship lance battery and "just" up the range, damage per shot, and fire rate so they can use that to kill whatever they want? In reality (or even in a semi-reasonable fictional universe) you have to make compromises when you design a weapon. Even if it could wipe out a planet by "colouring-in", unless it does it incredibly quickly anything that can move is just going to avoid the beam.
You have a point that the Tau wouldn't care so much about conquered worlds. And they probably would be willing to do terrible things for "the greater good". If we are considering DoW fluff, then if you win the Dark Crusade with the Tau they put all the humans in to single sex concentration camps and essentially wipe out the entire population anyway, so they obviously don't really care about other races.
I hope you aren't taking the things I say too personally Kelsik. We obviously have differing views on this but I'm enjoying the exchange.