Even if changing the ROM allows me to break the hardware, just by changing the ROM I don't lose my right to have the mobile repaired under free warranty. The manufacturer has to prove I managed to break the hardware only because I changed the ROM.
By the way, if you fuck up the CPU by overclocking it I guarantee they won't notice it because they will just throw the entire motherboard away. The subcontracted company that fixes your mobile (it's almost never the manufacturer) won't spend their time doing some CSI on it.