There is no net HP cost associated with changing stance on WAR. If you go Deliverance > Defiance > Deliverance, you do not end up with less HP than you started with. If you do an equivalent set of swaps with Grit or Shield Oath, you'll end up with an 1800 MP deficit in both cases.

The idea that Defiance doesn't provide an instantaneous benefit is false as well. The Convalescence effect (20% healing bonus) is instantaneous. The enmity gain bonus is instantaneous.

The true cost of Defiance, historically, was the fact that the damage penalty was steeper compared to other tanks (25%). This was partially offset by Unchained, but bear in mind that this ability previously cost 5 stacks (equivalent of 50 gauge).

With the advent of Stormblood, the developers attempted to homogenise costs. The Defiance penalty was reduced to 20%, but a gauge cost was added. While the gauge cost on stance swapping as well as the gauge cost on Unchained was removed due to complaints, the damage penalty on Defiance was not returned to its former level. So even if you felt that stance swapping on tanks was equitable in Heavensward (hint: it wasn't), the balance certainly tips in WAR's favour in the aftermath of the Stormblood powercreep.

A final point worth considering is that because gauge usage under Inner Release is now free, the potency cost of using gauge on an Inner Beast instead of a Fell Cleave outside of IR is significantly less than it was prior to 4.2. If you ever feel pressed to actually use IB, that is.

At the very least, a review of how tank stances function on all three tanks is in order. Things have changed.

One side note on enmity combos: one of the biggest costs to using your enmity combo is lost resources. WAR is unique in that you have gauge generation built into the enmity combo. PLD and DRK both lose MP (and in the case of DRK, blood) on top of having a larger relative potency loss when comparing the combos. So having more snap aggro tools on these jobs is important.