I've been doing a bit of tinkering with a Pi 3, both with and without a heatsink. My pi 3 correctly throttles when it gets to around 80C, as such I never saw the temperature rise much above 83C.
Looking at different articles, I notice different ways of measuring CPU clock speed, and I think that might have caused some confusion.
Either of these commands shows you the current CPU speed. If the Pi is fairly idle, it'll show a lower speed, if the processor is busy, it'll show it runnig at top speed.
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
BUT it won't show the effect of CPU speed throttling when the CPU is getting too hot.
To see that, you need to use this command
/opt/vc/bin/vcgencmd measure_clock arm
I wrote a short script, called Check.sh, thus :
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
/opt/vc/bin/vcgencmd measure_clock arm
/opt/vc/bin/vcgencmd measure_temp
cat /sys/class/thermal/thermal_zone0/temp
I then set sysbench running in the background, loading all 4 of the Pi 3's cores.
watch sudo ./Check.sh
let me watch the CPU speed and temperature climb.
Although I didn't plot or record the temperatures, I can confirm that :
Without a heatsink, the Pi 3 will quickly reach 80C under full CPU load (constant 100%)
With a heatsink, it takes longer to reach 80C, but will still reach it under constant 100% load.
Once the temperature reaches around 80C, the CPU frequency does get throttled back to prevent the temperature rising much higher.
With a desk fan positioned about a metre away blowing air in the Pi's vicinity, the Pi 3 with a heatsink never reached 80C under constant 100% load.
So in extreme cases perhaps a heatsink and a fan would be useful, athough it didn't seem to need much airflow to cool it.
But, realistically, is your Raspberry Pi 3 going to be under constant 100% CPU load?
I'm currently using the Pi 3, with the heatsink, NO fan, overclocked to 1300Mhz
, and the temperature is hovering around 56 - 60C.
Ian