Had a further think about this in the bath...
If you look at things differently, in the case of 300 TBN overbased calcium sulphonate, to get a 1 TBN boost, you're adding 591 ppm of 'carbonate' (presumably as carbonate ions). In the case of 400 TBN overbased magnesium sulphonate, to get that 1 TBN boost, you're adding 566 ppm of 'carbonate'. Given that I'm working with imperfect numbers pulled straight off Google, I rather suspect that 591 & 566 are effectively the same number and that it's the amount of carbonate present that determines how much TBN you get, and not how much metal.
I haven't done the calculations but I would probably expect this carbonate-to-TBN relationship to apply to all metallic detergents be they calcium, magnesium or sodium based or sulphonate, phenate, salicylate or calixerate based. If this is indeed the case, then for a PCMO, where acid neutralisation (as opposed to reaction product or soot dispersion) is your primary objective, then in terms of cost-effectiveness, you're probably best going for the highest TBN 'stuff' which uses the least amount of the cheapest form of soap.
I knew there was a reason I always liked 400 TBN magnesium sulphonate...I think I might at last have figured out why!!