At least in terms of common cuts, I'm fairly certain the component that causes the most "burn" is the methamphetamine itself. While everything is relative, I've typically had access to reasonably pure crystals, which when snorted are pretty high on the burn scale. In contrast, meth that's obviously been heavily cut with MSM (typically much cloudier shards) burns quite a bit less. I believe that's also true for isopropylbenzylamine (at least I've been told that) but I'm not sure that I've had drugs cut with it, though it appears to be becoming more popular than MSM.
As far as baking soda, it's extremely unlikely california methamphetamine is being cut with it as it is almost exclusively sold crystalized, something I suspect would be difficult to achieve with baking soda. I assume in AU you guys must be getting methamphetamine sulphate, or just a very poor quality of powdered methamphetamine for baking soda to be used as a cut. Anyway, while it was a long time ago, I once snorted a couple of lines of baking soda owing to confusion from some room mates smoking coke, and if it burned at all I'm sure it didn't burn anything like methamphetamine does. Broadly, all amphetamine related drugs I've snorted have burned to some degree or another, from methamphetamine to methcathione to 4-MMC and others, though methamphetamine definitely has been the most severe along those lines (comparing pretty pure examples of each)