So since so many factors look to be depending on the leadership abilities of the general involved, I think we might have accidentally stumbled across a balanced Vs. match. I know, everyone take a minute to sit down while you recover from the shock.
I'm still a bit skeptical on the 'looted Terran units' thing, though...Tarsonis had been occupied by the Zerg for a long time, and they pretty much trashed everything capable of fighting back when they took it in the first place. I honestly can't see the Orks finding enough equipment in salvageable condition to actually field an appreciable number of Looted Valkyries or Looted Siege Tanks; they'd be much more likely to get field-stripped and rebuilt into Battlewagons or Fighta-Bommas.
It's a toss-up, really. Some Orks would repair and add spikes to the siege tank, others would take every siege tank gun they can find and strap it to a single chassis while converting any salvagable siege tank chassis into battlewagons.
Luckily, ammunition and resources aren't a problem because the surface of Tarsonis is covered in Spikey Blue Rocks that can be converted quickly and easily into any metal at all.