I took a 4.7K gearscore guildie with me this morning (feral druid who had never been in ICC before at all) and he pulled over 5K on every boss fight, over 7K on Deathwhisper when I had him just stay on the boss the whole time. If your guild members are pulling 2.3K DPS (with the 20% buff) then either they are not good at playing their class, really confused during the fight, or really improperly geared.
Also, with the 20% buff, you should be able to down 8/12 in ICC10 man as long as people are doing what they're supposed to (avoid flames, kill proper adds, avoid maleable goo, etc...) and have proper rotations/gearing even with lower gear. Maybe not Putricide, but you should at least be able to get Festergut, Rotface, Blood Princes and Dreamwalker; so 9/12.
Honestly, just from reading your post it seems to me like it's the players more than their lack of gear. Tell them all to read up on the fights, watch videos, but most importantly, learn the intricacies of their classes and tighten up their rotations. 2.3K DPS was Naxx DPS, without a buff, and those gearscores are somewhere around 3-3.5K, not 4.5K.