What combination are you thinking about? Personally, I wouldn’t run a heavy spring and light buffer. Light buffers, like light bolt carriers, can result in poor reliability in austere or dirty conditions. Sure, one can tune the gas, but turning down gas doesn’t help reliability/robustness.
Also, what benefits are you reading about using a heavy spring with a light buffer? The heavy spring without a matching buffer will increase bolt closing speed and can increase wear.
The thing with a heavier spring is that it may slow bolt speed… but only on recoil. The heavier spring will cause a faster bolt speed on return.
But 100% with you on reliability concerns when one starts monkeying with edge case recoil setups.