Don't think I've seen many people discuss or upgrade. Not really a weak point after the tune.
keep the stock plugs, no reason to change them
:APR -stage 2 :BSH -Intake,Mounts,RSB,TB Pipe :Whiteline -A.L.K.,Endlinks
:Devils Own -DVC-30+Direct Port Injection :BC Racing -BR Coilovers :Flik-FTD :OSIR
:Neuspeed -Insert,Brake Lines:Eurojet -Ceramic Coated DP :DEFI :Unibrace :Tyrolsport :Bildon :Hawk
TSI FAQ: http://forums.vwvortex.com/showthread.php?4042736