redline water wetter has been excellent for me, and it did reduce my temps from 133o peak back to 121o, so 10-15o cooler sounds right.
Also, running straight distilled water (no coolant) takes another 8-10o out of the temps as well.
Having said all that, this was for full on 20 min race use (and I have other temp problems), I don't think you should have to run this stuff in a road car.....you have some other issue you need to fix.
Also be aware redline water wetter is not anti freeze, so if temps drop under 0 you risk splitting the radiator or cracking the block.