You won't see a ton of performance improvements with removing inline CSS, its just one thing you can do to improve performance.
The reason for removing inline CSS is to follow separation of concern principles. One example is imagine you are trying to fix a DIV, that just isn't styled correctly. You changed your CSS file, but its still behaving weird. You spend hours searching through the CSS file to see if its getting anything else applied, you search on forums, but no luck.
Then you check the HTML file, and notice you had an inline CSS. It could be pretty frustrating. Thats why you keep all styling in a stylesheet.