See the NGK Website info page:
http://www.ngksparkplugs.com/tech_support/spark_plugs/p2.asp

See the final note on the page, "As a general guideline, among identical spark plug types, the difference in tip temperature from one heat range to the next is approximately 70°C to 100°C"

(that's a 100-212 degree higher combustion chamber heat)

The way plugs work, the lower number is "hotter". So an 8 is one "step" hotter than the 9. Optimal range for plugs is 450 degrees to 870 degrees.

When you talk about burning extra fuel, my first question to you would be if your AFR (air-fuel ratio) is off. If you're running our FI Tuner Pro with a slip on and air filter, you're not likely to be dumping a ton more fuel in there, since a slip on isn't changing your flow enough to really necessitate a lot more fuel flow.

So yes, your engine will run quite a bit hotter, which you normally don't need to do unless you notice plug fouling, starting issues, misfires, negative changes in acceleration, etc. At that point you go with a hotter plug.

 

With the bike and mods you have you do not need hotter plugs. Stock with the stock heat range.

 

ok i'll stick with stock plugs. I don't have the Fi tuner yet, but i feel i need it because the idle/throttle isn't steady. Also i hear from other riders that a stock gixxer600 should easily lift the front wheel off the ground at full throttle, mine does not.

 

The only way to tell if you need a fuel controller is to get it on a dyno and see what the afr is across the board under load.

 

Pull your plugs and see how they are running, If signs of wet and rich, take it down a range. Or if someone rides behind you, or have them ride yours. Go up a decent hill and roll into the throttle. Smell fuel badly? Take it down a range. Im all stock, and I went from a CR9E to a CR8EIX and did it ever come to life. Going to run them for a few months and see how they do, if I need to try another range down.

Plugs dont Run hot or make the engine hotter, its a shorter electrode snout. less snout less ability for heat inside the chamber to transfer into the head and dissipate. So the heat is retained inside the chamber, helps with combustion. But get it running too warm in there an you will have lean issues, so its a real experiment with plugs to find the right range. Hell altitude can make a difference. This next winter Im likely going to buy a used engine and build up a motor. Coating piston tops, combustion dome and valve faces with ceramic has the same effect as a hotter plug. you want to keep all the head you can inside the chamber.