going from pre-cats to regular down pipes = big difference - you can feel the car breathing soooo much easier - turbos spool up MUCH faster - stronger exceleration feel
going from regular down pipes to split down pipes = noticable difference - the difference is mainly in when the turbos spool up - they spool up in my opinion around 500 RPM's sooner so you get to enjoy the hard pull exceleration for much longer :)))