One thing that almost always happens with bands is the second they get big enough to the point that they are touring extensively, the quality of all ensuing releases goes down. Most people attribute this to aging, selling out, or an exhaustion of inspiration. I think it has more to do with the amount of time available for song writing.
When a band releases their first few discs, they are working with material they had their whole lives to work on and refine. No doubt countless mediocre ideas were discarded in that time and only the most elite ideas persisted. That's why so often you see the trend of a first album being good, but unrefined as the band is just getting down the vibe of recording together and what their sound will be. Almost always a band's best effort is the 2nd or 3rd album - when they are still working with ideas they have had ages to polish AND they are not so big that they have to dedicate piles of time to performing, PR, etc..
Once a band hits that point that they are touring regularly, the way albums are written is like an amalgamation of little noodley ideas they come up with on the tour bus when practicing. Then they go into the studio at a set time for a set amount of time, and simply have to bang out whatever comes out in that session. That means almost everything they come up with makes it onto the record, even if halfway through the recording there is a sense that a song is really mediocre.
Think about Dream Theater. Got the kinks worked out on Day and Dream, hit it out of the park with Images and Words, which was based entirely on material they had all developed over there lives and while at Berklee. Then, after Pull me Under is a hit and they have to tour more and follow the more traditional route, a period of tapering off, until finally they reach a point that they are going to take the time to throw everything into one album and if it fails break up - out comes Scenes from a Memory, a brilliant work rivaling Images. Then, after the success of that album, every album has been alright, but nothing spectacular, mostly standard stuff that has a very definite feel of 'we're going into the studio on day X, will be done on day Y, then back to touring'.
Now, consider Symphony X, who followed a very similar path. They remained in obscurity and had all the time in the world until Divine Wings, which got the ball rolling. Then V is clearly the other brilliant work - but most of that material was hold over from an epic intended for Twilight that never ended up happening. As a result that material got developed and perfected far more than it otherwise would have been (which you can hear in the intricacies of Rullo's drumming or the synth arrangements, which are FAR FAR more carefully and cleverly construed than on all the other releases. Now, X is touring a ton and getting recognition, and you can hear that they do not have the kind of time to perfect songs now. PL has a distinct 'studio album' feel - there is nothing extremely innovative or complicated to it. Oculus is the closest, but that is a relatively simple composition in form and purpose, more so impressive because it takes advantage of newer technologies for samples.
You can probably think of countless other bands where this type of thing happens.
I guess my point being, I wouldn't so much call it a creative peak as 'got to a level of success that they simply don't have time to put out a perfected, meticulously crafted album like V these days. If Symphony X announced tomorrow that they were going into utter seclusion for the next 3 years to write a masterpiece of an album, I would forgo the live shows and tip my hat to the concept.