you might want to run something like “while true; do date && vcgencmd measure_temp && vcgencmd measure_clock arm && sleep 2; done;” in an ssh terminal when overclocking and running demanding games, if you overclock too high witouth proper cooling and i do not mean heatsinks alone then you might hit the temprature throtlling limit of 80-85 which lowers your cpu frequency and thus speed of the rpi3.
just blindy overclocking withouth checking this with demanding emulators is not that good of an idea.
about heatsinks in my opinion they do not really help, they seem to help but in almost all video’s i saw people don’t wait long enough to reach the temprature threshold. Eventually with heatsinks the same temp will be reached (when you are doing of course the same thing with the pi3) but it just takes a bit longer to reach that threshold.
The best thing is heatsink + fan a heatsink is not very effective without fresh air transported in and hot air transported out.
so the only positive side imo of heatsinks is that it takes more time to reach the same tempratures which depending on the action your doing could have an impact but if doing things like make -j4 when the 4 cpu’s are stressed 100% it doesn’t matter much just takes longer to reach the threshold.
my stock pi3 reaches 79°C while sitting in my picade playing tekken on ppsspp for a while so no small closed case but plently of room surrounding my pi3.
ah well i just wanted to point out it that it might be a good idea to monitor temprature and cpu frequency while overclocking and running demanding games and no 2 rpi3 are the same due to silicon diffrences which make some pi reach higher temps doing the same thing