I think part of the reason ADTOE started off the Mangini era strong is that much of the music, I assume, was birthed while MP was still in the band. I imagine they were also determined to put out a killer record as best as they could in the aftermath of MP leaving the band. The whole ordeal was probably a sort of adrenaline shot to the band. They also wrote it without MM, so they were still using MP's style as a template for the music. DT12, however, was the first one where they had to come up with new music with the new guy, and I think they hadn't settled into not working with MP anymore, creatively speaking.
Then they put out The Astonishing, which was a sort of a Dream Theater detour, and doesn't totally follow any growth or progression in the overall sound and style of the current lineup. TA just sounds like DT12, though not mastered as hot, with enhanced Disney acrobatics. DoT is the band finally finding their groove. It's mastered hot again, but the energy is high; however, I still think they're not totally there. DoT felt like they restrained themselves just a little too much, at least on Pale Blue Dot, and I'm just over the "here's how metal we are" songs at this point, like the first 3 tracks. It's not their strong suit IMO. I sort of feel like DT15 might be that album we've been waiting for since ADTOE, but I have low to moderate expectations.