Black Aluminum Heat-sink for Raspberry Pi 2/3B/3B+
Without any cooling, the CPU will reach temperatures around 80°C. Given that the Raspberry Pi's CPU throttles at just above 80°C to lower its temperature, if you're pushing the CPU hard then it's likely it will throttle.
Fan SHIM will give you 25-30°C of active cooling under high load. Its effective active cooling means that your Pi will likely never throttle its CPU.
Using a heatsink gives a small amount of headroom, and this will increase with larger heatsinks. Their passive cooling provides a buffer, meaning that short bursts of CPU activity will be absorbed, but extended high load CPU temperatures will only be a few degrees lower than without a heatsink at all.