Fixated on the word upgraded me
I have the Gradius Collection OST which has all 4 on it and its very interesting to listen to them in a row and see how its progressed, both the composition and the technology, its definitly evolved into more than the various versions of Outrun have (with the exception of Turbo Outrun going off @ 90 degrees) but, as you say, all versions are emmenently listenable to.
In fact the only thing I was disappointed with in Gradius V was the music, not that its bad mind, just that it doesnt touch base much with any of the previous ones (although thats about the only bad thing I can say about whats probably the best shmup ever made, oooo, controversy!)
The 2151 doesnt suck, it does a good job of doing all the horse work for a lot of mid 80's games, but I think the 2203 that Taito tended to use more was a bit nicer, or they were just generally better with it maybe
And the 2610 pwnz it or smth, Ninjawarriors forever
Also, mildly off subject, Konami are the only people I know to have gone backwards with audio technology, it took them a few years to catch up again with Gyruss (who cares if it was a one off)