I use mapp with XHP. As far as I know, the recipe for ruined steel either due to oxidation or over temperature is get steel where it shouldn't be and then keep it there. The longer it's in a bad place, the worse the result.
Chromium migrates from 800-1450F or something like that, and bad things happen (uniformity is lost, as well as corrosion resistance). Since I have no ability to heat it in an inert atmosphere, the objective is to take what's there and ruin it as little as possible. The old adage of changing temperatures slowly doesn't really apply because its temp at quench is 1850-2000, but it can be quenched any way you'd like to quench it. The bit of an iron is about 1.5-2 inches long and 2 x square and in a small can forge with mapp, you can get better temp control and even heating quick than you can with a big propane forge, and both of those are important, and then quench. It's turned out great every time except one instance where I was really pushing for higher temperature. I was just experimenting, and getting it hot and then oil quench is better than letting it sit in heat in open atmosphere chasing another 50-100 degrees to get a point harder out of the quench.
Temper is very forgiving in it. It only changes something like one point on the c scale for each 100F in tempering.
I don't think the problems in any of those other irons is a matter of anything other than wear and typical carbides - the only way you can get carbides as uniform as the PMs is PM - there are too many for them to disperse evenly like a plain steel. Crucible's chart showed that in terms of toughness. Wear resistance is unimproved by PM, but toughness is greatly, even with ideal process with non PM steel. A tungsten HSS would be interesting, but there are none (they would be costly). High tungsten steel was tried in razors (nothing makes a good razor other than plain steel - even O1 is marginal at best in a razor). I've had a few - they were fine, just slow sharpening. A straight razor at 16 degrees is the ultimate test of edge stability - you can shave with less than ideal, but you can really feel it. good edge stability will stand up to the strop and linen.
Someone on one of the razor forums sent me a message years ago about getting an S30V razor sharp. He was having fits with it (a strop will remove the very last bits of the edge on a lot of the high carbide steels). i told him to find the smallest diamond powder that he had around and progress to it, don't strop afterwards as the strop will knock out tiny bits of the edge (this happens on SG2, also, and probably a whole lot more) at the angle that a straight razor is honed, and send it back to the guy and tell him never to buy an S30V razor again. His razor cost him $700. It's not possible for it to be a good razor - S30V cannot tolerate a 16-18 degree angle and then pressure on a strop. Whoever the knife maker was who made it was fairly well known and I guess he was trying to get into straight razors, but he's going to need to learn to work with silver steel or something water hardening if he wants to make a straight razor that matches the old ones.
(I'd imagine the ward iron has some iron carbides in it - the O1 iron certainly does, but they don't have the same behavior as chromium and vanadium carbides - they stay in place better)
But, important to note, I don't think propane is quite enough to get an iron to temp for XHP - I've got a larger 2 burner propane forge for regular use and it can't get to high temps on a small space like the can forge can with mapp. Mapp can get a small area of steel to welding temperature pretty quickly if focused well enough - it's only a bit hotter than propane, but the amount counts.
(LN's A2 irons are the best I've seen, by the way. You can see on brent beach's page what the edge uniformity looks like on LV - not cryo treated - vs. LN - cryo treated. Edge longevity isn't really affected, but neatness is. I've seen LV quoted to say the same thing - they experimented with cryo treatment or tested edge to edge and found that either way, the edge gets through the same amount of wood). I tested a brand called IBC against the LN, too - i didn't do a huge sample, so it could just be one iron, but the IBC iron didn't keep as uniform of an edge as the LN iron.
it's one thing for carbides to be stable in a matrix, but another thing entirely to be able to have all of those carbides in a matrix when things grab and pull on the carbides and the matrix is wearing away.
(I can't remember now if I said it already - I consider the japanese iron performance to be defective, but the others perform as expected and I haven't seen better. Clifton's irons are forged and don't hold up better - I think they're as good as they're going to get - all of them ex. the japanese iron).
