With some steam pumps near the pre-heated exchanger (not directly adjacent), its consumption increases to 125 out of 103 steam/second. I'm assuming it isn't supposed to surpass that.

Thanks for this, I'll have a look.

Check the FAQ though, while a little wordy and complicated to follow, is dedicated to explain this very issue.
Just to be sure I'll playtest the mod again with pumps, adjacent and not, to make sure nothing is being "cheated".

Checked the FAQ. No mention of 100C, so the odd consumption amount is most likely from the unusual steam temperature. I only ever saw that in Realistic Fusion Power, where the heavy water boilers output steam at 100C, the minimum boiling point as the steam is a byproduct. Not sure if a fix even makes sense in this case if I understood the FAQ correctly.

Just to clarify:

The rate of fluid passing through the Pre-Heated Exchanger - so, rate of consumption and output - can actually reach up to 149 out of 103 steam/second.

I assume you have 125 because fluid throughput is reduced when pipes are put between fluid entities such as pumps and tanks

However, energy consumption to heat said fluid is a game constant, as is the unit of fluid it applies that heat to.
The game also assumes you will be using a 15°C fluid, not 165°C.
Essentially, each individual unit of fluid now has 150 degrees of space less to consume that heat.

To compensate, the Pre-Heated Exchanger processes a higher rate of fluid units in order to "use up" the same amount of energy.
If you divide the usual 485 into 335, you get 1.4477
Multiplied with 103, results in something just over 149.

I haven't playtested the following yet, but I assume it would also work:
Try mixing 165°C and 500°C steam, and feed that into the Pre-Heated Exchanger. You may find its processing rate much higher than 149 out of 103 steam per second, because now the average temperature of the input fluid is around 300, and not 165.
This is even less space per unit of fluid the Pre-Heated Exchanger can use up its unit of heat with, and again, will compensate.

Hope this helps 😅

Edit: I have updated the FAQ a little to provide some readability and less superfluous info.