Surely faster car goes(especially the modding an older csr) emissions are going to go over the normal?
They do, but that doesn't affect the road tax band. The band is determined by what the emissions were when it was registered. If you do something that puts the emmsions up then it's irrelevent for the road tax band - as for that matter if you do something to make it greener.
The Mk2 Focus RS crept into a lower band by one gram of CO2. Buy the RS500, which is already Mountuned as new, and you pay £200+ more road tax (it may be even more now). Buy the standard RS and get it mountuned, exactly the same car and you pay less in road tax.