Hi there, not sure if you have been to HPF yet but i saw your post and thought i would let you know that i went there for my GenV sedan after much research. mrt was my other option but it was quite difficult given where they're based!
i initially upgraded catback from Kleenflo Exhaust in Cranbourne - mainly for some sound and i personally like the oval look of the exhaust tips compared to round ones that many others offer - it worked in relation to sound and i'm more than happy with the look.
i then decided to upgrade power/torque which is where HPF came in. ecu tune and an upgrade from the turbo back to what Kleenflo had already done (i.e. bigger dump pipe and highflow cat). Torque increased by around 100Nm and power increased substatially down low - previous peak now comes in around 55km/h lower to speeds i actually do! Hpf seemed disappointed with the power increase up high but i wasn;t really concerned given i dont go out on the track to test it up at those speeds...and sound got even louder!
In terms of lowering, i got some bigger wheels with different offset and the rear tyres scrubbed - this is despite my research and many questions over more than a year! To be safe i ended up getting coilovers so i could control scrub and close the gap (i also rolled by rear guards). Never had any coilovers before but the pedders ones i ended up getting are going fine so far...very easy to adjust the ride stiffness through the boot or bonnet and no scrubbing - but obviously a harder ride!
I also have a whiteline rear swaybar and strut brace - these were put on before the coilovers and also improved handling/ride. other mods are mainly cosmetic.
if you're interested some photos are at
viewtopic.php?f=11&t=1357&p=193654&hilit=lgt#p193654good luck!