At the speeds you are hitting, you are correct that a formula board can't keep up on the same point of sail (broad reach). When GPSspeedsurfing.com kept a list of formula top speeds, very few achieved speeds over 30 knots (I think 34 was the top).
However, formula boards can run much deeper downwind than any slalom board and if on a dead downwind course, formula will get to the mark first. I have raced with most of the Crissy formula sailors at the US Open in Corpus Christi (all are better than me), so I know what they are capable of.
Ask them to pick a dead down wind course and race them some day. You will hit faster speeds, but you wont beat them to the mark.