DISCLAIMER: A Simple Mod I had an AI make to my own use (yeah, I'm no programmer). So, I will probably won't be able to keep it up-to-date or add any new features to it... If you are an actual modder, feel free to adopt this one and give it the proper care.

WHAT IT DOES:
As you can see in the thumbnail, it allows you to customize the power values of the vanilla Steam Engine and Steam Turbine.
It keeps the same steam intake and temperature demands.
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DanuaDolgoo (thank you!) Made considerable improvements to this mod, including Ukrainian translation,. Check it out. Link bellow:

https://mods.factorio.com/mod/CustomSteamPowerFromDanuaDolgoo

Anmr also made a version of this. It has Vietnamese translation.

https://mods.factorio.com/mod/heat-exchanger-balance

vs.1.0.4 - I had to correct the steam consumption.
The AI had it calculate the turbine's consumption (for example) as: (60/CustomizedPower)*(60/5.82)
That had the steam consumption climb very fast.
I changed the steam engine to be only the vanilla value, and the Turbine to be 60/6 (rounded up vanilla value).
That simplifies it... If I really knew how to program, I'd try to add a custom setting for steam consumption ratio/s, and change how add custom setting for how much the boiler and the heat exchanger can produce per second, so it can be even more customizable... alas, that's not the case.