Agree with above posters. The gain in sweetness with an FG of 1.020 vs 1.010 is going to be a much much greater deviation in the beer flavor, then the additional alcohol you'd have by letting it finish out where it will naturally will, likely somewhere in the 1.011 to 1.013 range.
If it is a super critical issue to you for some reason, the better bet would be dilluting the wort to 1.050-ish with pre-boiled water.
Primary: German Hef, Belgian IPA, Scottish 80, Belgian Dubbel
On Tap: Oatmeal Stout, Vanilla Oatmeal Stout, Belgian Dark Strong, Munich Dunkel, Dunkel Weizen, Oktoberfest, Bock, IPA, Black IPA, English IPA, Pale Ale
Using the mind to look for reality is delusion. Using your senses to look for reality is awareness.
"One time I was so desperate for a beer I snuck into the football stadium and ate the dirt under the bleachers." Homer Simpson
Life, Liberty, and the Pursuit of Hoppiness