I put on an 18 and have not looked back.
Have the Freak installed now (after being talked into it by Bluespoke!) so have lost no grunt (I dragged a mate on a stock TBA and led by a length every time) I do have the pipes etc etc as well.
It is important to note that there is not a whole lot of difference (compared to a rear sprocket change) check out / study the following handy web site ratio chart
http://wwwdropbears.com/u/utilities/sprocket.htm 
