Aftermarket matched manifolds
Platinum/Iridium spark plugs (more efficient spark than anything else)
Cat back exhaust system
remap etc etc.
You wont realise the full potential of any modifications without a proper remap though as fueling will change according to airflow - air filters/exhausts all change it, an engine will self learn to an extent....But not give the proper gains without the fueling curve and remapped parameters to match it.
You cannot remap the pre 2001 build V6 ECU's, they have to be rechipped. Remapping is the modern version of chipping - you don't do both (ok there is a scenario where you do, but for the majority this is true).
The output from the mapping is comparable whether its done via a remap of the existing EPROM or whether the data has been burnt onto a new chip and welded in.
I have Squadra ECU and 45mm intake runners. This increases low down torque, higher up BHP and deeper engine sound - but not cheap at about €550 but well worth it and would work even better with a good exhaust