Anything goes does not exist in a vacuum though, there's over-the-top regulations aiming at a specific goal (here, driving research on efficiency) and there's w

Anything goes does not exist in a vacuum though, there's over-the-top regulations aiming at a specific goal (here, driving research on efficiency) and there's what you want to achieve within a set of basic rules (e.g drag race, best lap, best 10 laps, max laps under 24h...). Take for example LMP, which used to go well above 400kph, but not so much car regulations as much as track regulations made that impossible/useless (e.g old Mulsanne Straight), designing top speed out of the cars because it simply was too dangerous. Mulsanne now has a brake zone from ~320kph down to ~100kph, much more humanly manageable. More downforce also means more drag, and for LMP's long running races this means too high a fuel consumption. Also, ground effects and closed body vs open wheels. Reliability (or lack thereof) is on the F1 side since LMP has to run hundreds of long laps, while a F1, even with "X races per engine" rules has to run continuously for a couple hours at most. Lift such a regulation and you could make a bloody efficient engine that self-destructs over a single race. LMP have a minimum regulated weight of ~900kg, and as every fellow driver knows, "weight is the enemy of performance". Barring that, and tuned to the problem, LMP would kick the pants off a F1 because F1 is (comparatively) insanely regulated. The beauty of F1 is that it achieves incredible performance in spite of suffering extremely limiting regulations. My bet is that lifting all regulations would quickly put the human as the sole limiting factor to performance, because the poor pilot of a guy would be on the verge of passing out at braking and inside every corner, unless they're on a balancing seat and have anti-g suits.