"than your character level" so no. Basically its up a feat compared to leadership, unless you take multiple ghosts in which case it has more HD restrictions. Is 3x15HD ghost cohorts > 1 19HD nonghost cohort? Sure you'd be down two feats but I don't know if that really makes it t3 material. Remember that I did stick it high in t4...
Eh? I thought that Leadership capped at 2 levels beneath you, and at 17th level regardless without Epic Leadership. I know there's a feat that gets you up to your level -1 instead of -2 (Close Cohort or something), but it's still limited by the chart's max level (17th at 25 Leadership score).
The biggest difference between T3 and T4, if I understand the tiers correctly, is versatility. Being able to do whatever you do (even with 6th level spells from an unimpressive list) and whatever your 1-3 incorporeal buddies are doing has to count for something. If it's one ghost, you have 17HD (so 9th level spells), and if it's 3, you have 15 HD each, so, say, one with 8th level spells, one doing whatever Telekinesis abuse TML can think up with 15 levels, and maybe one getting 9th level spells anyways via Ur-Priest or something. Having 45 extra class levels to play with has to count for a hell of a lot.