If I had my stars on they would probably be negative 15
Truth is I don't really care for the feature so I have it turned off...
Going up a pipe size should be fine. It certainly won't hurt.
I wouldn't worry about carbon build up. If your car runs well, I doubt its an issue...
I bought my magnaflows from the same place I had them installed. I would suggesst you go on magnaflow's site and find a retailer near you.
Otherwise, get in touch with Stazi (L8 Apexing Aussie) as he gets good deals on magnaflow products.