my suggestion: chill your starter to force flocculation, dump off most of the liquid over the yeast cake, let it reach pitching temperature, swirl to resuspend as much of the yeast as possible, and pitch a much smaller volume of liquid into your 5 gallons of wort. I use some of the cooled wort to resuspend the yeast cake after chill forcing flocculation.
There are many ways to do it...what you need to consider is the effect of altering the flavor profile of your beer when pitching a large starter. If your starter tastes like Miller (crap), I would be very wary of pitching it into my yummy wort. 2L into a five gallon batch is an ~10% addition (worse for 2L into 4.5 gallon)...therefore you have the potential of altering your flavor and aroma profile by ~10% (not scientifically sound reasoning here, just trying to make a point). You are asking a great question.
I usually make a starter media that is an exact replica of my beer (when I can)...wait for it to hit kraeusen, then pitch the whole thing...but I wouldn't do it with a 2L starter in a five gallon batch.
Run a quick forum search on "pitching starter volume", or "chilling starter"...I am sure you will find way more information than you need.
Good luck!!
PikledBill
