If I wanted to get a rMBP and planned on using an external monitor when at home, how big of a difference will the 750m graphics be? I read in the forum that hooking up an external monitor kicks the 750m card on, but then also read some people only had the Iris pro and it worked fine, the macbook just got a little hotter than w/o the monitor. How much difference would the 750m rMBP make over the Iris pro?