The TIM has been tested, as you said, to last through 7 years of bake and power cycles. Downclocking the processor may help but we cannot guarantee the correct functionality either from the processor.
Our best recommendation is to keep the default settings on the processor and renew the TIM when necessary.
Is the 7 years estimated on the basis that the computer will be turned off for (for instance) 8 hours a day, is it based on (for instance) an expected idling time of 50% for the CPU, or is the estimation for the lifetime of the TIM based off of a constant usage scenario?
And what would be the recommended TIM to use as a replacement, if it did need replacing in a number of years? Would a TIM such as Arctic MX4 be suitable?
The processor along with the TIM has been tested under constant usage of heavy applications. I am afraid to say that we cannot guarantee the correct functionality of a third party TIM but we can do it when using the Intel TIM with the P/N G15816-001.
My best recommendation is to contact the manufacturer of that TIM you mentioned and confirm with them if it is capable of cooling down enough the processor you have.