Is there a performance hit for creating Extension methods that operate off the type 'object'?

I have a set of extension methods that I regularly use for various UI tasks. I typically define them to run off of type object, even though inside of them I'm typically converting them to string types.

public static string FormatSomething(this object o)
{
     if( o != null )
     {
          string s = o.ToString();
          /// do the work and return something.
     }
     // return something else or empty string.

}

The main reason I use type object and not string is to save myself in the UI from having to do <%#Eval("Phone").ToString().FormatSomething()%> when I can do <%#Eval("Phone").FormatSomething()%> instead.

So, is it fine from performance standpoint to create all the extension methods on object, or should I convert them to be string (or relevant) types based on what the extension method is doing?