A positive displacement supercharger (which is what we have) moves a fixed amount of air for every revolution of the input shaft. Compressing air creates heat. The amount of heat created is a function of the amount of compression and the efficiency of the supercharger. The M90, while more efficient than a Roots type blower, is still fairly inefficient.
In addition, as you attempt to compress more and more air, it takes more and more power to turn this input shaft.
So as SC input shaft RPM increases there is an increase in:
1) Air moving through the blower
2) Heat transfered to the air
3) HP stolen from the engine
Eventually you get to a inputshaft RPM that adds so much heat that the amount of energy needed to turn the shaft, is greater than the energy that can be added to the engine via the SC.
The stock setup of the SC engine uses a very small and inneficient Air to Air intercooler to attempt to reduce the heat left in the air before it enters the engine. This works pretty well stock. But it quickly can reach it's limit of transfering heat.
Why I say all of the above is so that you understand that by placing a smaller pulley on the SC input shaft, you are going to turn the SC input shaft faster for a known engine RPM than if it had the stock pulley on it. That means that if the Intercooler became heat soaked at 5100rpm, it now may become heat soaked at 4700rpm. If the HP required to turn the SC was 40hp at 3000rpm, that 40hp will now be needed at 2600rpm.
These are just numbers I'm pulling out of my hat, but the theory is the same. Turning the SC faster (which is what a 5% or 10% pulley does) moves the power increase earlier in the power band, but also moves the power draw, and the heat generation earlier into the power band.
Thus a 5% pulley will give you a little kick and likely not drag the bad aspects of heat and HP robbery too low into the power band. Make sure your exhaust is free flowing and you'll be good to go.
A 10% pulley will give you a kick but it will drag many of the bad aspects into the lower reaches of the powerband. Thus, at least in my mind, a 10% pulley necessitates some other upgrades to be fully taken advantage of and not have it make things worse. I.e. better intercooler, bigger throttle body, bigger MAF.