They definitely should not slow down right now. They just replaced a key member, and while Mangini has won over most of the fans, he still hasn't really proven himself in the studio, so a new album, with MM as an actual contributing song writer, should be delivered in short order to secure his position as the new drummer. A Mike Portnoy-less DT still has a lot to prove to the fans, and ADTOE was just the first step. They now have to show that MM is more then just a flashy hired gun. He has to show he is a drummer who can be creative in the studio as well as insanely talented at live shows.
Also, the band has to prove that they can continue to maintain high chart positions and album sales with the new lineup. One of the reasons why MP's timing for his hiatus was so poor was due to the fact that DT had just cracked the Top 10 for the first time in their careers. To take a hiatus just when the mainstream was finally starting to open up would have been incredibly foolish. With MM, the band not only had a second Top 10 album (albeit slightly lower then the last MP album), but also a Grammy nomination. Dream Theater should continue their drive into the mainstream while the momentum is still with them, and see just how far they can go in the music business.
One last thing. Although it may be an outside chance, one of the few things that could push the band's career even further would be for them to make a true masterpiece of an album, on par with I&W or SFAM. I don't know how likely this is, but with the recent roster shake-up, the creative juices a probably flowing is quantities unseen in a very long time. ADTOE, despite some new elements, was overall a very safe album for the band to make. If they really push the envelope, they could have a third opus in the bag, and such an album would not only secure MM's position in the band, but also further the position of the band as a whole in the wider world of music, and may be one of the few things that could help Dream Theater to finally crack the Top 5, and maybe even have a # 1 album. It is a long shot, but I feel that it is definately in the realm of possibility.
Clearly, DT still has more room to rise, and they should continue pushing up further until they hit an obvious popularity ceiling. When that happens, then
DT can consider taking a break, slowing down their output, and turning their attention towards other endeavours outside the band. I personally feel that if the band is smart, the next two or three albums (at least) will still be made on their current two year cycle of albums and tours. After that, then the band probably should consider taking a break, whether MM works out or not, whether they break further into the mainstream or not, and whether they make another grand masterpiece or not. By that point, with 13 or 14 albums under their belts and all the band members in their 50's, the band will be reaching pretty far into seniority as a rock band, and scaling back their activities as Dream Theater may be a necessity for the good of the band.
EDIT: also, a lot of what I wrote in this post:http://www.dreamtheaterforums.org/boards/index.php?topic=33986.msg1396026#msg1396026
has relevance to this topic, as well as it's native thread.