I'm using the Canson Plantine paper with the K7 Selenium in one 3880 and it is nice stuff. Not a super bright white, more of a warm tone, but it does look nice.
I've tried multiple passes of the GO. What it does is fill in the texture of the paper's surface. One is often enough. I've tried 3-4 passes of GO and it smooths it out too much in my opinion, even with lower levels of say 20,000 instead of the normal 30,000 in QTR. Plus, it gets very expensive to use the stuff up so soon as you do use a lot of it in a single pass verses several other tanks of ink mixed for a single pass on one print. I had to buy it in the larger bottles too since I wasted too much playing with it. Nice stuff though.