with respect to unmodified encounters

Taken in isolation (with respect to unmodified encounters), this is true. Also, if the player AND the mobs are treated with the same conversion rate, this is true.

Like everyone else, I can only presume what Blizzard are intending, but I suspect that all exponentially powerful mobs will be down-converted into a linear scale appropriate for their level and the ilvl of level-appropriate players.
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The exponential problem is this (converting bosses into approximate gear requirement tiers, and using 10H where possible):

C'thun had an ilvl that was 1.47 times the character level, Illidan had 2.16 times, Lich King had a 3.39 multiplier, Deathwing's ilvl was 4.89 times level 85, and Garrosh goes over-the-top (as usual) with a 6.29 multiplier.