Would it be possible to have the highspeed curve, line, and s curve pipes be able to go up or down? I am only seeing them go horizontally except for the gain tube, which only goes up 1 m at a time.

We can discuss this for sure, but I intentionally did not create HS tubes going up or down.
This was a design decision, based on the fact that they go fast, which is for long distances.
You can think of it as items having to fight gravity when going up or down and thus HS Tubes are not possible vertically.

If it is gravity inspired, then down would make sense while up would be as you said. If not that's ok too. I still love this mod. :)

Items going down would need to be stopped and if gravity accelerates them things would go boom.

And only allowing down not up would only confuse players.

But viaducts are for players, not for items/fluids. Holding up items/fluids on a pipe can cause jam and crafting request confusion. -1 on this.

You could, instead, use the method Thermal Dynamics does with its long-distance viaducts whereby transitioning from a regular viaduct to a long-distance one requires a couple of seconds to build up energy before moving forward at the higher speed.

I'd say that would just be part of the design process. Perhaps high-speed pipes don't route for requests by crafting pipes and modules meaning you'd need a local storage depot for machines or crafting tables. The crafting result would travel via the high-speed pipe, just not the ingredients.

that sounds too complicated

Moved to https://github.com/RS485/LogisticsPipes/wiki/Feature-Requests