Known plenty of tuners to ditch devil kits due to reliability of the kits.
I terms of mixture, you would use both, water cools, meth will allow advancing, going 100% to one or the other won't benefit you, meth on its own will give the advance, but no cooling factor that the water would give, so the mixture option is where you need to go to achieve the desired benefit.
Also don't get meth mixed up with meth
