you really only have the MEEK or plazmaman off the shelf aftermarket items that will be pretty jucha direct fit...
some people upgrade the IC to vr4 or evo spec, thewn adapt their piping to suit, but the gsr piping isnt much to write home about
as above, if sourced for the right price the factory evo option is a great upgrade... you will however need to upgrade your throttle body (which is highly recomended in conjuction with an intake manifold porting) as there is no way in hell that the evo pipe will clamp up to the gsr tb...
any IC upgrade from the stock gsr bread-loaf is going to produce better results...