The Creature Comfort seat is my favorite kayak seat (of the diferent types I have used, including in some random hard and soft seat designs in "plastic" kayaks). I have the Creature Comfort in my Shearwater Sport.  I've never had any of those sit on top type kayaks with big fishing-type seats.  Those are probably nice for the purpose they are designed.

As to your question about the Creature Comfort, however, if you take a look at the Mill Creek gallery pictures, you'll see that the back height on the Creature Comfort isn't really very high.  In my Shearwater Sport it is only about the same height as the standard kit backband would have been.  In the gallery, if you look at pictures of the boats with the stock wood kit seat and compare that with the pictures of the Creature Comfort seat installed in the Mill Creek (see slides 17 and 18) you'll see that the back heights are about the same.  I won't try to guess which might be more comfortable for you, and I'll note that you probably could place the Creature Comfort on top of the wooden kit seat, but still wouldn't gain much back height.  Also, if adding one on top of the other, remember that the higher your butt is from the bottom of the boat, the lower the stability.  Maybe not a big deal in the relatively stable Mill Creek.

Have you looked at the Folding Cane Seat carried for sale by CLC?  That's a possiblility.  Looks like you might even be able to shave 1 or 2 inches off of the lower portion of the seat frame to lower your CG, if desired - And that might be something you could consider doing only after trying it first without modification.  That seat has always looked so nice to me, and at a relatively good price, that I've considered choosing what I might build to put it in.  Not a kayak, for sure, but looks like a good idea and distinct possibility for the Mill Creek.

   Thanks for your thoughts Bubblehead. The Cane Seat is definitely something to consider. In fact for the price it's worth a gamble. I'll let you know how it works out!

Thanks Again, SB

   A higher seat back will interfere with your paddling quite a bit . 

   Thanks for the input. Most of my paddling experience has been in canoes with some  sit on top kayak. I'm trying to get it right the first time without having to redo things especially like building a seat. I can see how a high back could interfere so now I'm back to first base. I guess I'll have to  "dance with the one I brought" and build the original seat which I do like the look and will match the front seat. I can always modify later if needed.