My advice: brew your kit exactly according to the directions. You'll have a mush easier time coolling those 2 gallons than you would a larger boil, and your recipe is formulated for a 2 galllon boil.
Here's why it matters in recipe formulation: the higher the specific gravity (concentration of sugars in the wort), the lower the efficiency of extraction of bittering compounds from the hops.
So, a larger boil means a lower gravity wort, which means you'll extract more bittering compounds from the same amount ot hops.
You can use the free Beer Recipator software to play with these numbers
But any improvement you could make in your kit's quality would be negligable. Just brew it according to the directions and concentrate on getting comfortable with the process.