If Blizzard were to decide that a 1.25 multiple

If Blizzard were to decide that a 1.25 multipler of item level and MOB level were appropriate, then Deathwing, for example, would be a 106.25 ilvl boss, meaning that a level-appropriate group would require an average item level of 106 for the encounter (10H).
wow gold farming
Now, if Blizzard also decide that the PLAYER'S multipler was 1.5, this would mean that a L85 downconverted player would have an ilvl of 127.5, a L90 of 135, and L100 of 150.

Obviously, there's a universe of maths I haven't dealt with there, but I suspect that the linear versus exponential scaling will be solved in down-conversion by giving a smaller multiplier to the mobs than is given to the player. Still a linear multiplier, though.