Normally, I'd think this would be out of scope this late into the D7 cycle. But, we very recently got #653622: Make the '__' pattern for theme suggestions easier to use in, which brings the '__' suggestion convention out of obscurity and into core. Also, we recently got #538164: Comment body as field in, which raises the priority of improving performance of theme hooks implemented as templates. One thing that helps is a static variable cache in drupal_discover_template() (#667030: Optimize drupal_discover_template()), but with this issue, we can get rid of that function entirely, and have even better performance. Finally, we have (had it in D6 too, nothing new) an annoying clobbering WTF: #405496: Can we change the convention for template_files override from hyphen to something else in order to avoid clobbering? because '-' is used to delimit both multiple word hook names and suggestion suffixes.
So, on balance, I think this patch is worth it. I'll post benchmarks soon.
Just to clarify, the net effect of this patch is that instead of node-TYPE.tpl.php, you would use node--TYPE.tpl.php.