"I don’t think you’d be far off the mark to believe that the boost at UPenn will be significant in the ED round provided you have the stats to make you otherwise competitive. "
This is the right way to look at it. Legacy/ED is mostly a tie-breaker.
Penn has a middle ACT range of 33-35. If you have a 32, legacy/ED is probably not going to get it done. Because Penn has many thousands of applicants who show up with a 34 or 35 ACT.
But when Penn looks at its big pile of 34 or 35 ACT applicants, the legacy/ED apps are much more likely to get picked. When playing a game that produces many ties, having a tie-breaker is HUGE.