Suggestion: add settings to allow modifing train speed/braking force/acceleration/weight/grid size etc.

I feel BTS already provides a good variety of entities and tiers for all the rolling stocks in game, while keeping it balanced. Is there a particular case more settings are needed?

Keeping BTS balanced is good. But in general, players are the last ones decide how balanced their games are.
While some ppl think BTS should consume more fuel, some ppl still think BTS should be faster, and some think both. There is no end trying to balance the whole picture.

Just try thinking that you are not making BTS imbalanced, but giving more options to BTS users.

I feel BTS already provides a good variety of entities and tiers for all the rolling stocks in game, while keeping it balanced. Is there a particular case more settings are needed?

A particular case: mods that change train base speed can affect BTS. However, the speed difference between tiers is fixed numbers.

Example: when a player changes a vanillia train speed from 250kph to 500kph, Tier 2 speed should be increased by 100% (not 250kph) too.

What modifies the vanilla train speed from 250 to 500?

TL;DR - Settings to tweak each tier will be added when I'll have a bit of time to work on it & refactor the code :)

Long answer: more settings = more edge cases to manage. Giving 100% of freedom is cool, but has to be maintained then... Just like in vanilla and other overhaul mods there's not always the possibility to edit every single aspect of it, I feel like BTS already offers a large variety and upgrades to vanilla prototypes, as well as compatibility to other popular mods.
I wish that mod that modifies locomotive's speed from 250kph to 500kph did a better job at it and modified BTS's prototypes too. Maybe you can submit a request to that mod's page & ask to integrate better their functionalities. At the moment I'm literally doing night shifts to keep up with work, I would like to add this functionality and keep it "as open as possible", bug free, and efficient; but I cant offer a solid timeframe in which this will be accomplished :)

Edit: I will accept PRs to the source on GitHub if any :)

What modifies the vanilla train speed from 250 to 500?
Config Train Specifications (Modified Update) by zmounce

Config Train Specifications (Modified Update) by zmounce
Modify the Locomotive Speed, Cargo Wagon Size and Fluid Wagon Size. Now work on all modded trains.

Maybe you should report the bug to them, since appears the are supporting it