k03s has slighty larger turbines which will hold boost a bit higher up the rev range so you can get a bit more power out of them with an ecu remap.
Can't remember off the top of my head why vw switched to smaller ported heads, something to do with gas velocities i think. A larger ported head will generally flow better but I wouldn't worry about that too much unless you're going for silly power with a big turbo upgrade.
