Can I use 1.35V DIMMs on a 1.2V CPU?

Probably not. Running RAM at 1.35V overvolts the CPU’s memory controller, which is rated for 1.2V max. This can cause instability, corruption, or degrade the CPU over time. If your sticks have an XMP/EXPO profile that drops to 1.2V at lower speeds, that’s fine; otherwise, don’t force it. The performance gain isn’t worth replacing a CPU.