The Devil's Saddle is good. It looks nice, it makes Roach black and it has cool effects, but it's performance is slightly underwhelming.
It is especially weak considering that it was literally made by the Devil himself to make Geralt as swift as the wind and it's only 10% faster than regular saddles made in Toussaint.
So i made it slightly better!
I've only seen one mod that attempted to do this so far and it skewed the Caparison of Lament completely in the opposite direction, making it utterly OP.
My goal was to make the Demonic Saddle feel like a worthwhile reward after completing such an amazing adventure while remaining balanced.

If it conflicts with anything, make sure the line 19 in def_item_horse.xml says min="120" in both Bundle\dlc\ep1\data\gameplay\items and items_plus folders, just like so:
18    <ability name="DevilSaddle">
19        <stamina                        type="add"    min="120"                />

Works in both regular and NG+.
The mod is provided AS IS, i take no responsibility for what you do on your own system and i probably lack the knowledge to help you out with any issues...
But otherwise go ahead and use it, it works great for me!
Tested on Polish GOG GOTY version 1.32.
Huge thanks to ScoutBr0 and lucky_the_luke from the Witcher 3 Discord server who thought me how to use the WolvenKit!