@Brimor The 8k spring will allow the engine to rev higher before the clutch engages the clutch bell. It will give you more take of power.
The 8k doesn't have draw backs in my opinion and is the ideal spring for most of the engines out there up to 34cc.
When you go to 9.5k spring, that's were you may run into issues like over heating, burnt or broken clutches depending on the engine.
Overall 8k is the way to go if you still have the stock 5-6k spring. You'll love the take off.
I personally use 9.5k on all of my engines and have had only one instance of overheating clutch and that was with my Stock Zenoah g320rc. Which is weird as it has yet to happen with my full mod OBR g340 reed.