While that does show a difference, what it fails to show is whether or not getting a gear set of high spirit items is worth it.
I was under the impression that the point was not to acquire a spirit set, but rather that the spirit currently on your gear could be better utilized. Regardless of the amount of spirit you currently have, having the 70% instead of 55% of it turned into crit rating would always be an upgrade.
On a separate note, I fail to see why the molten armor buff that's already on you would be changed by merely changing two pieces of gear. I could see why changing specs might cause such behavior since buffs are usually wiped clean after changing specs although in live this doesn't apply to molten armor. Essentially all that's happening is that you have Molten Armor A which can only be cast with two piece t9 equipped, and Molten Armor B which is only cast when you don't have the t9 equipped. The % of spirit to crit should not dynamically change depending on whether you have the t9 equipped or not.
As an easily testable example, cast any shield in game that's dependent upon spell power. Let's assume that it absorbs 10,000 damage. Even if you take off all your gear, that shield will continue to absorb 10,000 damage for its duration. The amount of absorption would only change if you were to recast the shield altogether after losing or gaining spell power (ignoring the "a more powerful spell is active" annoyance).