Instead of updating every single web.config of your website I think it is better to just update once and for all by updating machine.config. What you say ?
Seems like you can't add a system.webServer node on the machine.config, but you could add it to applicationHost.config (source: http://www.iis.net/learn/get-started/planning-your-iis-archi... ) so yes, that would be faster. You have to make sure that none of your web.config files override that setting though!