It's not just the construction at LGA, the main highway out of JFK, the VanWyck expressway is under construction. On your travel date will the Mets be playing at home? Will US Tennis Open be occurring? Both of these effect traffic to LGA.
When you land check traffic reports, if there are huge delays use mass transit: JFK AirTran to Jamaica, E train to 74th St, Q70 to LGA will bypass road issues.