Wow. 1st of all, there aren't that many differences between SG or SF. What you are asking Burks and Sap to do is play two widely different positions. As a PG you are asked to defend lightning quick players, bring the ball up the court, initiate the offense, look for others,etc. There are a lot of responsibilities PG's have to handle, which would take away from Burks' best attributes.
For Millsap, he would have to play out on the perimeter. Yes he can shoots well for a PF, but with quicker defenders closing out on him will he be as good? Also, he would be away from the basket, where he thrives on offensive rebounds (you can't crash the boards as hard as SF because you have to worry about getting back on defense) and he would be a liability guarding more athletic SF's. Favors can play PF or C because he is a physical and athletic freak. Millsap is not.