Jesus....lol, blah blah blah. Bunch a ladies. Lol. To actually answer the OP question.
11-14 GT/CS wheels are 19" X 8.5"
YES, 275/40/19's fit!
They may well physically mount to the wheel and hold air, but it is a fitment outside industry recommendations. On the low side, which can lead to slightly greater sidewall heating in use and less precise handling. There may be ABS and/or AdvanceTrak consequences, as those systems are calibrated with the OE tires and any differences between OE front and OE rear in mind.
On the matter of understeer, it's there all the time and can even be expressed in terms of the difference between front and rear tire slip angles. More slip angle up front (narrower tires or less cornering-performance-oriented tires) is the definition of understeer. And vice-versa for oversteer. Maybe you don't notice this at normal cornering levels - the difference might only be a fraction of a degree - but as cornering g's increase so does this difference in slip angle. You may never plan on cornering at 0.8g, but occasionally poo happens and you might find yourself way up there, Think about it, and ask yourself if you really want to piggyback a second surprise on top of the one that's provoking the emergency maneuver in the first place.
You can crutch a tire stagger understeer with less understeerish suspension tuning, though this puts the tires and the suspension working at cross purposes as you approach the limits. Doesn't make a lot of sense to me.
'08 GT coupe, 5M, wheels, tires, pads, fluid, a few suspension mods . . . still almost stock height
'19 WRX, 6M (the "family sedan")
'01 Maxima 20AE, 5M (the spare and occasional winter driver)