I have been building SFF systems since 2005,so yes thermal constraints are important.
Instead of making excuses,think and maybe you forget people tend to play games for hours too. Maybe in your world people only run games for 10 minutes.
Look at JC2 for example:
http://img.hexus.net/v2/graphics_cards/nvidia/Titan/f343d/JC.png
15% to 20% would push the card down to GTX680 level.
A well known French website did the test,and look,it boosts more than what Nvidia says. So those performance figures are dependent on good cooling. It also appears the card in throttling under Anno2070 on a
test bench.
If performance drops a decent amount it comes into the same league as cheaper cards which won't throttle in a SFF PC. Hence the user can get the same performance from a cheaper card in a SFF PC.
If you don't get into your head,thats your problem.
Complain to that website then,why they actually bothered to do some additional testing.