There are two main points we're trying to address with this change:
Raiding in 10- and 25-player dungeons already provides the same item level rewards for everyone. Engaging in the logistics of the larger raid format, however, we feel should be rewarded and the slight increase in chances to receive a Thunderforged item in 25-player is our effort to strike a better balance between both raid sizes.
The addition of Thunderforged items also have benefits for both 10- and 25-player raids. For instance, if a raid group is stuck on boss #5 and their members have all the viable upgrades from previous bosses, going through those bosses again to work on #5 can now bring added bonuses. We think this makes these efforts much more fun and rewarding.
Unless slight increased drop is 50% more, it isn't going to remotely come close to fixing the 25man problem. Slight in blizzard terms reads as 10%. That isn't enough incentive. At best that is an average ilvl upgrade of .5-1ilvl over the tier versus 10man. Not good enough.
And please let us not forget the issue in the first place, that thunderforged is an idiotic structure. Make the 6 ilvl increase be via a upgrade token like firelands, otherwise the system will be broken, and it will hinder a lot more than it helps.