How far can I lower my 2011 sedan with 20x10 275-30-20 and 20x9 245-35-20 before I rub or have to modify the fenders?

 

You should also mention the offsets of the wheels and suspension mods.

 

35 front 40 rear, I want to just get spring

 

You can drop it w/ no issues using those wheels/offsets w/ those 245/275 tires. Depending on what springs you go with you'll probably also need at least a rear camber/toe kit.
If you were to go w/ a deep drop spring such as a Tein S Tech or H&R you may get some rubbing of the rear fender liner but that should be it.

 

I was just wanting the eibach pro 1.5 1.2 drop

 

You'll be fine but I would recommend at least the rear camber/toe kit.