Most start with an exhaust upgrade for a deeper louder sound and Triumph make an identical straight through cat free open pipe that looks identical to the stock cans but performs.
Some also go aftermarket for stock headers back and others go the whole hog with headers too from various suppliers:
3 here in Oz
@ozrider of the forum makes a 'snakey' version, as do CES and REBAND;
1 in NZ (Viking Exhaust by another member
@Paul Bryant );
1 in USA (Carpenter); and
another in the UK (TTS).
Next is an air intake upgrade to a RamAir filter kit which comes with air temp detector cable extension (for relocation at the throttle bodies), a filter to cover crankcase breather bypass, and instructions on how to remove all the restrictive plenums and under seat airbox etc.
Other options are 4-5 versions of pods from K&N and pods from RamAir and Unifilter too.