RRL, are you over inflating when studding? It helps to balloon the tire up, stud it then when you let air out the rubber contracts tighter around the studs. You can also dip them in rubber cement before screwing them in. You are correct on making sure to use angles on studs but it certainly has to be forward tilted from the top side. Then the outer two rows that get double studed, you need to face one so it kicks outward to keep the bike from spinning out while the other is on a 15-20* angle equally inward and forward. I'm kind of pissed it took me a few years to get it dialed, but the pros aren't exactley leaving tires uncovered for you to really see what they're doing.
As far as the 20" tire, it's what I've ran for 5 years now. I'm happy with it, but after trying my stud combo on some 18's last year I'm lindor torn. The 18" puts more weight/pressure in a smaller spot so it hooks up on the straights. The 20" spreads the weight so the tire slips a bit more, but it corners much much better.