They would not be very affective against ISIL ground forces given that the F-15s are an air to air platform, which means they carry air-to-air weapons.

“In the latest sign of growing tensions between the US and Russia in Syria, the US is sending planes equipped only with air-to-air weaponry to the region, David Axe reports for The Daily Beast. As Axe notes, Russia’s the only potential US adversary in Syria with its own combat aircraft.”

http://www.businessinsider.com/us-moving-f-15cs-to-fight-over-syria-2015-11