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to be honest looking at how the pipe has been squeezed by the turbo flange, and the fact that you don't want to go for "mental horse power" you are best sticking to your original headers
to be honest mate the power out put you ae looking for is easily achieved with the headers and up pipe you already have, and there is no CAT in you up-pipe
Whats the advantages of porting the standard items over tubular headers?
IMO...
Ported headers will fit easier and maintain or improve spool slightly over standard. Sort of raising the original bar.
Tubulars' different pipe lengths will change the way the engine wants to behave. Likelihood is that even after tuning you'll lose a bit of spool and maybe some midrange but gain at the top end. Also bear in mind that a lot of header designs originate in the US market and are aimed at 2.5litre engines.