On the surface the idea with the new raid system that tier 1 items are ML26, tier 2 items are ML27 and tier3 items ML28 seems reasonable. This means you can use the lesser items sooner.
But I have a feeling the effect instead will be that the lower tier items will largely go unused. Going from level 26 to 28 is a matter of a relative few hours for most people - so organising a full set of gear with effects that match up, purchasing augments etc. at level 26 is somewhat pointless as the time spent to organise and store the gear most likely outweights the time you are likely to wear the gear and certainly the time/value you get from wearing it from 26 to 28.
Thus, unless the ML26 is actually worth wearing beyond level 28 and most importantly at level 30, there will be a much lessened reason to actively acquire said gear.
On the other hand - if the gear from all these raids where made the same level AND the effects on the tier 1 and tier 2 gear upgraded to (almost) the same value as the gear from KT/Baba/CoS, then it would be an entirely different matter. Now, if the aim is to give people on the TR train more time to use such gear then simply make it all ML26 but upgrade the tier 1 gear to a +17 level, the tier 2 gear to a +18 level and keep the tier 3 at the +19/20 level. Similarly, lower Slavers gear to ML26 without lowering the stats and likewise lower all other named gear above ML26 to ML26 while keeping the values intact.
In short - accept that most people wont bother farming and maintaining specialized gearsets at every few levels when those go by relatively fast - and therefore make all the gear ML26 so people can focus on setting up a powerful gearset at that level knowing they get to use it both for leveling from 26 to 30 and while being 30. The feats at 28, 29 and 30 still makes a level 30 significantly stronger but less so making grouping more flexible as well.
Will this be powercreep? Well, yes, it will, but imho very insignificantly so and possibly just mean that level 26-30 is done in a more fun way rather than EN/EH dailies or Orchard/TH slayers.
If nothing is done I fear that the large update to add the tier 1 and tier 2 raids into a joint system will fall somewhat flat as few will actually care enough for the loot from these raids as its outdated within a few levels/hours of play.