do you have a crappy/dead motherboard to make a few test? if so, use that to determine the time/heat/ distance you would need to melt the solder. usse the dead board, point the heat gun on some smaller parts, watch the solder - when it looks more "shiny" - it should be molten (don't forget to keep an eye on the time it took to melt!) - then go to the next part - a bga ic - there should be a few on a common motherboad - make the same thing - but you won't see anything whats under the ic - but you should keep an eye on the surrounding parts (shiny!) - in case of a dead board - you could also try to "move" the ic a little bit (way less than a mm!!) - if the solder is molten - it should move slightly (don't try that on a board that should be repaired - it would most likely kill it) - same thing as before - keep an eye on the time! - now you sould be able to determine the heat an the time it would take to reflow the chip on the ps3.
i always use soldering flux for those kind of repair!