That's effectively the opposite of shortest path. If you really want the best performance you'd set up a line of microwave links.. I'm not sure how much latency the hops would add but if you have fast enough gear, it's probably not enough to justify going all the way up and back down.
The ionosphere is not all that far away. Radio travels at about 0.97c in the atmosphere. At shortwave frequencies the ionospheric E layer provides the best reflection IIRC. The E layer's altitude is less than 100 miles (60-90mi) but can bounce a signal hundreds of miles horizontally.
If you want to reach a receiver a hundred miles away an HF bounce off the E layer would get there in 0.659ms (if my math is right). If you had a 100 mile long fiber optic cable your signal would get there in 0.813ms.
Microwave repeaters can be very fast so long as you've got good positioning. If you managed a straight line of repeaters to a receiver you might be able to beat the HF path. For the best results microwave repeaters shouldn't be too far apart so for the hundred mile trip you'd want several repeaters. Each is going to add something like 150ns latency. This is cumulative with the 0.97c air latency. And it's only to a single receiver.
So microwave repeaters would beat HF sometimes, depending on topography, to a single receiver. I think HF would end up winning out more often than not and wouldn't require the infrastructure of the microwave repeaters. It would make siting receivers much easier and would likely allow receiving stations to optimize siting for the last mile to the trading systems.
That being said I don't think HFTs should be able to abuse HF by broadcasting things not usable by the general public. If they want to cut their latency they can use some of their hojillions of dollars to build microwave infrastructure or pay everyone for use of the HF airwaves they want to make money with.
>the shortest path
That's effectively the opposite of shortest path. If you really want the best performance you'd set up a line of microwave links.. I'm not sure how much latency the hops would add but if you have fast enough gear, it's probably not enough to justify going all the way up and back down.