Welcome to the boards.
Well... I'm with SV, mostly. I don't see the dramatic change that you do - other than with his appearance (and that's always changing). I agree with "Meatplow" being an easy fit on Core, and think "Silvergun Superman" or "Unglued" would have worked on there, as well.
But, I also think that around the time Purple came out is when Weiland really began to develop as an artist; a true genius of his craft. He was able to use the success of Core to branch out into some more experimental sounds, and he drew from the criticisms that first album received as inspiration for new material ("Kitchenware & Candybars"). Core was just a launching pad - a starting place. It gave them enough of a jump that they were able to move forward and really come into their own.
STP is one of those bands that has always been able to evolve and change their sound, while still remaining true to themselves and their roots. They never come off as a cheap imitation, and always put their heart and soul into what they do. Personally, I think this is one of the things that makes this band TRULY remarkable. Its the reason I first fell in love with them almost 13 years ago, and it's the reason they haven't left my heart since.
If you're looking for more information on STP's formation, Rik has a great band bio and timeline, which you can find right here:
http://www.belowempty.com/bandinfo.php?s=bio