Can I modify the Performance Recorder template?

Yes you can

The template file isn’t altogether useful, I mean, whilst it does help, it could be a lot, lot more informative.

And modifying is as simple as editing the template file in Tableau.

But where is it?

In the Tableau directory of course

	C:\Program Files\Tableau\<your Tableau version>\Performance
	/Library/Tableau/<your Tableau version>/Performance