hard to tell how it's going to sound, really. needless to say Petrucci and Portnoy were the major creative influences in the band. it will be interesting to see what their new drummer brings to the table, i can't imagine they will bring someone on to just get given a sheet and play. that's what LaBrie is for.
i'll nominate Portnoy, too. he's the best i've heard and seen perform in person. his full Dream Theater kit commands the utmost respect just by itself. most of his kits are less drumkits, and more percussion command centres.
his Mirage Monster setup:
... and it's still not the biggest kit i've seen him play. i think it was the dvd "Liquid Drum Theater" in which i saw his biggest kit - pretty sure it would be his full recording kit, with full gong and everything included. it was about 270+ degrees around him. absurd... and the real absurd things is he has a f**king purpose for every single piece, and knows how and when to use them.
Portnoy is a tough act to beat, in both his ability to compose and ability to play. he is certainly amongst the very elite.