"The only catch is, in order for the tab-focusing feature to work, you have to close the tab using your assigned hotkey."

This throws me off right away. There are so many shortcuts keys already from the OS and normally I'm pretty good at memorizing (some if not all of) them. Those extra gadgets are supposed to help people to make their life easier, but shouldn't force people to memorize more things IN ORDER TO use them properly. I'll pass.

For what it's worth, you could use the Keyboard prefpane of System Preferences to assign some hotkey other than Command-W to Safari's Close Tab command, and then use Command-W for Tabkeys's tab closing command. That way, you would get the benefit described in this hint without having to memorize another hotkey, even one as simple as "W".