go to
www.clublexus.com and go to GS section. tons of info on the posibility of your car.
hahah $5000 for a custom turbo on a GS4.. I WISH!! but i dont think so.. there are only a couple of people i think that has custom boosted (twinturbo) V8s and they spent over $30KUSD on the kit alone.
brand new intakes for the GS4 will cost you starting at around $250USD but if you get the SRT intake and ECU it will cost you around $500USD. However you will lose torque when you install an intake on the GS.
Headers are not being mass produced yet but there is currently a group buy for custom fabricated ones. No word on actual HP gains yet.
Best bang for your buck as far as perfomance mods go would be an upgraded TC (torque converter) they can shave off .5 seconds off your acceleration times. They sell for about $850USD. LSD kit is also available by RMM or TRD.
a couple of companies makes superchargers for the GS4. Rod millen and SRT (3 stages). they range from $5000USD - $12000USD. Some people are running intercooled blower setups and are producing some nice numbers.
as far as suspension mods go dont go for JIC magic (rip off) TEIN coilovers are the most popular. The HA and FLEX series . Upgraded Sway bars (TRD, Daizen, intrax) make a huge difference in the handling of the GS especially when coupled with upper strut bars. Steering ECU is also available to stiffen up the loose steering.
Supra TT brakes work well with the GS and its a good budget brake kit. Kits from ROTORA and BREMBO are also available at a higher cost.
A NICE set of 19-20" wheels assuming "nice" being brands like HRE, VOLK RACING, RACING HART, WORK etc will definitely cost more than $1500-$2000 for a set. They can go as high as $1200-$2500 + USD per wheel. go to
www.hrewheels.com and you see what i mean.. *drool*.
I can go on.. but this is getting too long already.. but the bottom line is There are LOTS of things you can do with your GS...
it all depends how deep your wallet is..