Is this going to lead to faster compile times? Faster register allocation...
Very few compilers actually use vertex coloring for register allocation
Totally. The hard part isn't coloring (you can use simple heuristics to get a decent register assignment), rather, it's figuring out which registers to spill (don't spill registers in hot loops! and a million other things!).
No.
In SSA, the graphs are chordal, so were already easily colorable (relatively).
Outside of SSA, this is not true, but the coloring is still not the hard part, it's the easy part.