Besides the stock carpet like material, has anyone used a different type of material like vinyl or anything else?

I had my targa top sprayed with rhino liner both inside & out. Looks great.

I've a burled aluminum dash panel and @MoveQik Mike Wills and a few others have replaced the back pad with Perlon

The carpet cuts down on noise a little bit.

sure does, and it is easy and cheap to install and also keeps the matching and factory look. Vw did not put it in the early production cars, but found it a nice sound deadening improvement, so it was then standard in all 914s

You could try dynomat but probably overkill.

Could make your roof a bit more bit weighty Mark,a roof made like these will sweat ,the carpets there to stop that as they do on the inside of fiberglass boats and camper vans...I know a little about that