Sig says "London".... you are working in Italy??
> you also pay for cos-pi (I am not sure if it is written like this in english)
Probably "power factor", PF.
Simple loads such as motors draw a sine-wave of current but this is not necessarily in-phase with the sine wave of voltage. Typical motors pull peak current slightly after peak voltage, "lagging phase angle".
If the angle reaches 90 degrees (peak voltage at zero current and vice-versa) then no real power is consumed, but large energy is sloshing from generator to customer and back to generator, heating the transmission system and that is real power and real cost.
A loaded motor will have phase angle perhaps 5 to 45 degrees, drawing some real power to do its work and some slosh due to reactance, induction, and other issues. Nobody cares about a few window-fans; factories with hundreds of horsepower of motors can be very bad. It is economic to add huge capacitors to cancel the lagging phase and present a near-resistive load to the power company.
This could be a factor on your air conditioning, but that's considered in A/C design. Small units will run bad phase but are too small to warrant PF metering. I assume huge systems that might trigger PF penalties consider PF in design.
But most of your audio loads are DC power supplies. These historically did not draw a sine-wave of current, instead they draw two spikes of current per cycle. This is an issue in power systems, and sometimes a PF number is assigned. However especially in Europe, PFC (power factor correction) is common on all larger (and many smaller) switching supplies. You are unlikely to run into PF penalties with a studio.
> balanced mains, which I have seen (or heard) in operation and the results are rather amazing...
This means different things in different contexts.
In 3-phase power, the savings from running 3/4 wires is lost if most of the load falls on 2 wires. A large 3ph motor naturally pulls the same on all 3 phases. But lighting is usually 1 phase. Consider a 3-story building, lights on each floor to one of the three incoming phases. When all lights are on, load may be well balanced. In a recession, 2/3rd of workers laid-off and upper floors left dark, the load is seriously unbalanced.
But around audio geeks, there is a different concept of "balanced power".
The customer wiring must be grounded, so that when a 13,000 Volt distribution line falls on a customer line, the customer is not exposed to 13,000 Volts. The customer ground holds induced voltage low and the near-short soon clears (burns-out) the fault.
We universally ground one of the power wires. Usually the one called Neutral.
I will switch to talking 230V.
In normal house wiring, the outlet has one hole at "zero" Volts (relative to ground, conduit, concrete, steel, plumbing, etc) and the other is 230 Volts away.
This is equivalent to "unbalanced audio", guitar and RCA plugs.
We could, instead, use a center-tapped 230V winding. One outlet hole is at 115V and the other is 115V the other way. The load gets the full 230V. However each line is half the voltage (to ground etc) and in a cable the field from the two leads tends to cancel a few cm away from the cable. This is similar to XLR or TRS balanced audio lines.
This needs a specialized service transformer. (It may not be enough to re-wire a 0V-115V-230V winding, because the two 115V segments may not be exactly the same, and the stray capacitance will usually be different.) There is no call for plan this except around very sensitive audio-frequency systems. (Not even telephone, because 50Hz is below telephone response.) It is not actually banned by US NEC, but is so unusual that electric inspectors are baffled and would normally reject it. Most recent gear will take center-tap power, but older gear may have assumed a grounded pin, and occasionally some boutique gear is just wired wrong in a way that works fine with normal unbalancd power but burns-up other gear when balanced powered.
At best, I would use this at racks and console, with 2KW boxes, not general wall outlets.