Tommy
When I started, my alto had a Yamaha 5C. Like the 6C, they are plastic. Later I upgraded to a Selmer C* and, like all the ebonite ones I tried, the sound was much warmer. I just liked the Selmer best at the time and like Yamaha Mad, preferred it to the C**.
The 5C is 67thou tip opening, the 6C is 71.
The Selmer C* is 67, the C** is 71
So If you went for the C* you'd be going for smaller opening. If you're used to the wider opening already you might not find the C** so different in terms of air support I guess, but you might notice the tone. You really have to try them for yourself.
YC