Hmm, ya know, if Jim Matheos is at least equal with JP as a songwriter
He is!
you have to wonder whether OSI would give DT a run for their money if it weren't for Kevin Moore's monotone vocals. If DT reunited with Moore and Portnoy and it wound up sounded like a more melodic OSI, I could really dig it.
Kevin's vocals are perfect for OSI's style IMO - a singer like JLB would be totally out of place on songs like Wind Won't Howl and For Nothing.
I don't know if you've heard Fates Warning, but you should give them a try. Disconnected is musically similar to the first OSI album and even features KM on keys. Their new album doesn't include any keys, but a lot of the riffs are OSIish and one song has lyrics written by Kevin. Ray Alder sings on both records and his vocals are more melodic and high-pitched than Kevin's.
I don't agree with the point that Kevin wouldn't fit with DT musically any more. The guy still writes heavy, interesting, and somewhat progressive music. His flavor of those musical influences could really add to what DT has been doing, and most importantly, he's another completely capable full-song writer. The songwriting and melodic choices could really use some variety after ADTOE and DT12, in my opinion.
I think the heaviness in OSI comes mainly from Jim, while Kevin comes up with a lot of the more spacey and Chroma Keyish ideas. The reason I don't think Kevin would fit in modern DT is that his songwriting relies on atmosphere, whereas DT have always been about technicality - they've gone in opposite directions. A mesh of their styles would be interesting, but I'm not sure either side would be willing to compromise that much. On top of that, KM isn't really a team player: one of the reasons he left DT and hasn't joined any full-time band since his departure is that he prefers to work alone. Of course he's collaborating with Jim in OSI, but that's just a duo and they have a good songwriting chemistry.
I always thought the first OSI album sounded more like DT than anything else KM has made. Dunno if that's cause MP was more involved with those sessions than the second time or if it's a coincidence but the compositions always felt like they had more of that progressive metal edge on the first album. I think OSI has been getting a bit more electronic/experimental (which is a very KM kinda thing to do in fairness).
Well, Jim started writing for the first OSI album before Kevin joined, and at first the songs were much longer and closer to standard prog metal, but once Kevin came in he started chopping the songs up, adding spoken word samples and electronics etc. On all the later albums he's been involved right from the start, so his influence on the material has been stronger, which has led to a more electronic sound.