I'm in the position of knowing a group of people who import whole leaf tobacco (and often seeing just what they are receiving).
Although the packages are almost always 'heat treated' I've also see a reasonable number of those that aren't.
Although I've never seen the treatment process I believe that it's done in largish appliance, very similar to a microwave oven. It is also of significance to know that the packages or the box they are in are not opened. They do this twice a week - on Tuesdays and Thursdays.
There is no doubt that the process changes the tobacco. It certainly darkens it.
However, in my opinion, I don't think that the inherent quality is changed or degraded by it.
Some of my acquaintances, myself included, actually think the heat improves it - particularly the aroma. The 'mild burning' which you mention is probably exactly what I'm talking about - and you mention it 'smells quite nice'. As you probably know, some people, in processing their tobacco, 'toast' it. The end result of that is pretty similar to what heat treatment does for 'free'.
The only drawback with stuff that has been treated is that the leaves are usually sort of 'stuck together' in the package. They don't just easily separate like they usually do in an untreated lot. It takes a bit of time (a day or two) using water spray, steam, or a humid day to get them pliable enough to handle.
It is easy to blame heat treatment for what comes out of the package - for making it brittle or 'rubbishy',etc. But I've seen plenty of leaf that could be described as this come out of untreated bags.
So, it's not all bad. The other thing that is quite positive is that they seem to have got their act together a bit better at MPI. Not too long ago it was often taking eight weeks or more from go to whoa. Just lately packages seem to be getting through from ordering to delivery in just over a month.