A couple of quick notes:
#1 The Oldbood/Scar-vet cowboy (on a cold one) is far more effective than the Carno build. The Carno is too vulnerable to artillery, shooting, magic and even close combat while the Cowboy can safely hide in a unit of cold ones and is much more defensive. The Oldblood/Scar-vet cowboy is our single greatest unit. Cheap and effective.
#2 While the Warpfire Braziers makes Thanquol/Boneripper significantly more dangerous in close combat, it does come with a very serious weakness... namely it confers Flaming Attacks to CC. That means that a Cowboy can be very easily equipped to have a 2+ Ward Save in combat.
What about an Oldblood with the following kit [GW, Cold One, Dragonhelm, Dawnstone, OTS] 226 points
The Oldblood is S7 and has 1+ re-rollable armour save and a 2+ ward save vs fire.
Thanquol/Boneripper will need [4's to hit][2's to wound, re-rollable] and you'll have [5+ re-rollable armour save] [2+ ward] (wounds caused benefit from d3 multiplier)
The Oldblood will need [3's to hit + PF][2's to wound] and Thanquol will only have a 4+ ward save where successful wards must be re-rolled.