1. I was wondering at which point the vcore can be pushed until it void the warranty ? Is there any way Intel's know about the overclock I did ?
2. Also, is there any preference between going for a lower multi and higher qpi or full 20 times lower qpi ? 3. What is the max temp at 100% ? 4. What is the safe temp ?
1. Intel would not know as long as you don't tell them and there is no significant sign of burn out. I would stay within 1.40V which is the Maximum Vcore specification from Intel.
2. There is almost no effect on the Vcore. However, you will need higher Vtt for higher QPI.
For example, you need much higher Vtt for 200x18 than 180x20. In addition, you will DEFINITELY exceed the Absolute Maximum Vtt at 200 BCLK.
3. I don't have my i5 at 4GHz, so I can't tell you.
4. 72.7C is the safe case temperature according to Intel and therefore you should be fine with any core temperature less than (72.7+2)C.