If I decide to create a new application server to host the Central Admin, Index, and/or Query roles will I get a noticeable performance boost overall for 100+ users or is this more just a security design consideration
Is it easy enough to simply add the application server later if performance is an issues I believe I can remove the Central Admin role from my current server using the Config Wizard and add it to my application server easy enough

Changing from a 2 server to 3 server architecture....
Propellorhead
Personally if your going to add that extra server I would move the Index to that and keep that box for index only.. Let you front ends run the search and queries and balance that load. The Central Admin is recomended to be put on an App Server but it works fine on the FrontEnd or WFE... its more of a security seperation from the sites that is being recomended.
Hope this helps