So here's the skinny on rotors, stay the hell away from anything drilled. They're only good for stress fractures and you don't want those. Slotted is usually fine, I had a set on my LT, they tend to have a better initial bite when it's wet out since there is that channel for water to escape. That being said, more metal means more braking before fade happens, it takes energy to slow your car down. I'm running blanks on my LNF, though I might be looking at slotted next time around because of how wet it gets here in the 11 months it rains.
For the conversion you have 2 options, you can get an entire rear axle and swap it in or get mounting brackets. I recall there being something funky about the e-brake. Look around the forum, I recall a guy with an Ion doing one.
Edit:
Found it, sadly there are no pictures.
http://www.chevycobaltforum.com/forum/f20/how-rear-disc-conversion-336/