I heard running a 75 shot you could get away with 25s but I wouldnt even do that...just stick with the 24s and I have read Platinum dont mix well with our cars.
Dont use platinum cuz its harder than the iron cylinder walls where as copper is softer. And if you blow the tip off it will ruin your engine whereas the copper wont.
Platinum no good for spray either. Colder plugs wont cost you any power...how did you figure that. The reason for different plugs is that you want to be in the optimum temp range. Too cold a plug and it will foul up...to hot a plug and you may cause pre-ignition as the tip is hot enough to ignite the mixture w/o spark or worst case melt the plug. Check your plugs (after not using the spray for a while, as nitrous has a tendency to clean things in the cylinder) and if they are fouled, your using too cold of a plug. If not then your fine. Also if the plug is too cold and you dont wanna change them...figure out how long it takes to foul them and just spray the car before that interval and they will be clean again or at least cleaner.
Isn't 25 stock? The 24s which is what I run, is only ONE step colder. Most nitrous kits ask for TWO steps colder. The only power I left on the table N/A was my timing was set a 6 degrees for 3 years.