[RICE RACING]

I am not an expert as I am still learning this myself but would offer you this advise based off my own experience.

IF the car can take your regular petrol and the tuning is considered acceptable (say not too rich or retarded, and its adequately intercooled *~45deg C charge temp @ full power*) then I would say to you that you will always loose or retain very similar power by the addition of (water, water methanol, or methanol alone) *meth alone has potential to increase power due to latent cooling effect and the fact its the easiest to ignite*.

Keeping major variable like boost pressure constant....You will only make gains thus if your:

*AFR is set too rich
*Air temp is higher than acceptable
*Timing is retarded too much

As Richard I think said, when your Ignition system settings (voltage, current, spark gap, plugs used etc) are capable to ignite the WI mixtures you will typically see gains in power once fuel mixture is optimized, air temperature stabilized & timing added. Obviously the biggest variable is BOOST PRESSURE and the addition of correct levels of WI (again ref Richards notes) will open up a new world of power based on this.

How much extra power???? (Great question)

Its relative to what you can ignite (see ignition tips)
In my own tests 50/50 by mass of water to methanol works well but you can still have too much of this (each car is different! and needs to be tested properly on road and measure the gains) in my own case I needed to use the correct AFR, right mixture, and tuned flow rate to give 21% gains on power with only 8% of this increase given by boost increase alone (SQRT(higherboost/lowerboost). So the Water/Methanol mixture for me to get it to work right needed much individual experimentation to get the best result possible.

What is the practical limit? only you can find out in your own case, if you look towards others tests (very detailed) with a magneto based ignition system you can/could replicate their results I am sure and expect gains of 50%+ more power ***Assuming your turbocharger can deliver the required flow and the higher boost pressures it will allow you to run***

Hope this covers it for you.? (with fine tuning and being realistic I say 10%+ power is achievable)