I figured this was some strange interaction with some of my mods so I went to a clean forge install with only:

Forge 10.13.2.1291
Big Reactors 0.4.2A2
COFH Core 3.0.0RC7-211

The power multiplier config option behaves strangely. I tested the following settings:

Control: Default config
262 RF/t

Power Multiplier: 2

Expected: 524RF/t
Got: 1.03 KiRF/t (~4x)

Mult: 3

Expected: 786 Rf/t
Got: 2.31 KiRF/t (~8x)

Mult: 4

Expected: 1048 Rf/t
Got: 4.1 KiRF/t (~15x)

Mult: 10

Expected: 2.6 KiRF/t
Got: 26 KiRF/t (~100x)

It looks like the config is somehow exponential.

Are you changing just powerProductionMultiplier, or both powerProductionMultiplier and reactorPowerProductionMultiplier? Both are applied, so if you're changing both, then your results make sense.

I can confirm this issue. It works now diferently than in versions before.
In all tests I have reactorPowerProductionMultiplier=1.0

I have a maximum efficiency BR turbine in my base (32 ludicrite coils, 80 blades, feeding it with 2000mb/t of steam)
When I have set the powerProductionMultiplier=1.0 the energy I am getting is: 28kRF/t which is correct.
When I set powerProductionMultiplier=0.6 in previous versions (before 0.4.2A2) i was getting 17kRF/t (that is indeed 60% of the 28k), but now in 0.4.2A2 I get only 9kRF/t (which is about 30%).
When I set the powerProductionMultiplier=0.8 I am at 17kRF (but it should be higher, since this is not 80%).

powerProductionMultiplier is applied both to the reactor and to the turbine, hence why you're seeing exponential results. Raising powerProductionMultiplier raises both the amount of raw energy generated by the reactor (and thus raising the amount of steam produced) and also raises the amount of energy generated by a turbine from a unit of steam.

It sounds like what you really want is to change either reactorPowerProductionMultiplier or turbinePowerProductionMultiplier, but not both.

I was only changing PowerProductionMultiplier, and I was not using steam turbines. Just a basic 3x3 reactor in a field with nothing else. Is that also intended?

No, that's odd. Also, in the future, please explain your setup in your initial bug report.

Oh, wow.

powerProductionMultiplier = (float)BRConfig.CONFIGURATION.get("General", "powerProductionMultiplier", 1.0f, "A multiplier for balancing overall power production from Big Reactors. Defaults to 1.").getDouble(1.0);
reactorPowerProductionMultiplier = (float)BRConfig.CONFIGURATION.get("General", "powerProductionMultiplier", 1.0f, "A multiplier for balancing reactor power production. Stacks with powerProductionMultiplier. Defaults to 1.").getDouble(1.0);
turbinePowerProductionMultiplier = (float)BRConfig.CONFIGURATION.get("General", "powerProductionMultiplier", 1.0f, "A multiplier for balancing turbine power production. Stacks with powerProductionMultiplier. Defaults to 1.").getDouble(1.0);

You're seeing exponential results because it's reading powerProductionMultiplier's value in for all of the multipliers. Herp a derp. Fixing.