At 175 there are several ATG that could give him a run for his money. Spinks and Foster were mentionned before, maybe Ward too. It's hard to figure out accurately how the likes of old timers such as Charles or Moore, with a blend of power and craft, would overcome Roy's speed advantage. I guess you can't write them out.
At 168, dunno whom this side of Ward... and I'd favor Roy in a close contest. Maybe prime Calzaghe's volume would work, but I think Roy stops him.
At 160 it takes some frigging speed. Maybe Leonard. Hard to discard SRR either. Hearns's reach and power can make it interesting.