Good question. I don't know. I've never sailed an e.g. IS127/137. So I can't compare.
Perhaps Remi or Jean-Marc can answer this one.
My guess is that on reaching courses and typical PWA downwind slalom courses, there will be not much in it. Probably the US needs less pumping. On the other hand, in slightly upwind courses it could be a different story.
If I compare the US and Formula in upwind conditions, I find there is still quite a difference. Formula rules without any doubt.
IS87 / IS101 / IS122 / US147