That would not be enough. it would alow the i tag to be in the editor but would not change the behaviour of the italic button. it would still create em:s. I would create a new plugin (using the same visual class and such) that wraps curent selected content in an i tag. this should be a pretty simple plugin to make and would be a good practise plugin also.
Editing the current plugin directly to instead make i tags is not recommended. since i think its a part of the advanced template somewhere or so in tinymce, its not a good place to change things considering future updates to tinyMCE/episerver.
If you look at the specification for tiny mce it says "/ Separates element synonymous. The first element is the one that will be output. " and I can verify that it works just like that so by using the code above you will get i instead of em.
I would like to modify the Tiny MCE editor used in edit mode to generate i-elements instead of em:s when the editor clicks clicks the italics tool button. Any pointers on how to do that?