Put it this way. Say a Barbarian is wielding an Executioner's Axe, is in a rage, and decides to use Rage Strike. He burns a 29th level Daily to do so, and crits. That means 9[W], which is a d12, so that's 108 damage, right off the bat. Then you want to add 27 d12's on top of that? Going by the idea that high crit is multiplied by the [W] of the power, that's what we would get with the Executioner's Axe. On average that would come out to a grand total of 310.5 points of damage (factoring in the Brutal 2) before you even roll the magic item crit dice. That's just ridiculously powerful.
The property works as it was written. It's 1[W] additional damage per tier, and that's it. It's not 1x[W]. This still gives high crit weapons quite a damage boost over time though, especially once you factor in the weapon mastery feats that increase your crit range. Additonally, a large number of magic items only add d6's for crit dice, whereas many of the high crit weapons use larger dice than that. This means that with many weapons, the high crit dice value will be almost equal to the crit dice value from the magic items. 3d12 Brutal 2, for example, will be better on average than the 6d6 from a +6 magic item. Even with lower dice, like d8's from a Scimitar, you're still getting a lot of extra damage.