Cell said Trunks SSJG3 easily surpass him in raw power, so there should be a considerable difference based on this. But Trunks is still not stronger than warm up Cell by a slandslide. I remember @Papasmurf pointing out Piccolo saying "it's even greater than Cell", rather than saying "WAY greater" indicates it's not by a very wide margin. I think 1.2~1.25x seems fair.