There's a problem with your calculations, you don't take into account Freecure procs.

Every Freecure proc means that you save 442 Mana and you gain 250 potency heal. In a perfect world you would have 5 procs every 36 cures, but as we all know RNG can troll you and you can get 0 procs or 11 procs. I'm just going to take 5 procs because that would be the average if infinite people would do that experiment you proposed.

Assize scales with Piety, and that means the more MP the more Assize will restore (my WHM has 15000 MP at ilvl209 and that's because most of my gear gives me Piety) and we will have more MP the better gear we get.

Cure potency = 400 x 31 + 650 x 5 = 12400 + 3250 = 15650 total heal potency
Cure MP cost = 442 x 31 = 13702 MP spent
Assize MP regen = 10% x 12500 (it's a more accurate MP value for healers that have some piety) = 1250 MP recovered

Benefic potency = 400 x 36 = 14,400 total heal potency
Benefic MP cost = 354 x 36 = 12,744 MP spent

15650 heal potency for 12452 MP vs 14400 heal potency for 12744 spent.

Of course freecure procs are random, but with so many heals it's nearly impossible not to get at least 1 or 2 procs.