I don't know, are we really 100 % sure Portnoy was behind all that's been accredited to him, e.g. the heaviness? I used to be too, but since I've heard LaBrie's solo career, I'm definitely less sure than before... I also think that JP's just as big a metalhead as Mike is/was, so I'm definitely not absolutely convinced SC and BC&SL wouldn't have sounded the same/very similar even without Mike - I'm more inclined to think it was simply something they had to get out of their system (and I'm glad they did, since SC is my favourite). The fact MP's been doing the "tough guy" vocals might have been simply that he was the most willing/comfortable with it to do that...
We're definitely not sure about it, but personally the telling clues for me are
1. The fact that almost every time DT tackled a heavier style in a way that's not fitting them at all, it was almost always accompanied by cheesy backing vocals by MP.
2. Adding to the first reason, there was this whole apologetic post by MP on his forum back when BCSL came out, explaining that he thought the original way JLB sang the (what later became) growling section was awkward, then he tried all sorts of combinations and growling styles before settling on the final cut.
3. MP was the most vocal person to talk (in general, but particularly about) about their "inspiration corner"
4. The immediate bands he went on to when contemplating to and actually quitting DT were A7X and Adrenaline Mob, both kinda reminiscent of SC and BCSL's awkward moments (to me at least).
5. The awkward heaviness was completely gone the very next album they released after he quit.
I'm sure that the other members are not just "sheeps" who blindly followed everything Portnoy said and that it wouldn't have happened if they didn't like it as well, but it really seems to me like he was the main force behind it. I may be wrong, of course.
1. Ignoring completely that almost every time DT tackled ANY style, it was almost always accompanied by cheesy backing vocals by MP. That's what he did (and that is not a knock; I always kind of liked the contrast with James' voice).
2. As I understand it, he tried many combinations, not limited to growling, yet that was what supported the music. Blame the music (which, I understand was largely written by Petrucci) if you are set on "blaming" anything.
3. You've already answered your own point: MP was the most vocal person to talk IN GENERAL.
4. This is revisionist history. He had already indicated his time with A7X was coming to an end before he officially quit DT, and he announced Flying Colors around the same time as announcing AMob. Plus, if you look at the first releases after leaving they are: AMob (EP), Neal Morse, Yellow Matter Custard, AMob (LP), Flying Colors, Morse-Portnoy-George, Neal Morse, Prog World Orchestra. Portnoy didn't solely focus on metal, though what he did was compartmentalize his offerings (which I do have a problem with). But there was a LOT of prog in that group. And if you look at the BC&SL covers - which Portnoy selected for the most part - there is a lot more prog than metal in there. Finally, Petrucci is the guy that almost always drops a Metallica riff or part in his solo spots, not Portnoy, so it's not really fair to point to MP as the "METAL PUSHER".
5. I don't completely disagree with the "awkward heaviness", but it is still there in the two non-MP albums, I think it just comes out differently. Those two albums are so dense with guitar, they barely breathe, and that is more Petrucci than MP (well, ALL Petrucci, since MP isn't on those albums). I think it is a manifestation of the same idea.