Have a read of this:
http://gtfour.supras.org.nz/chassis%20dynamics.htm
I have had Bilsteins for several years on my two GT-Fours. I got the rears off a Team Toyota Italia rally car so they bolted straight on. I used normal street inserts for the from revalved to fit.
I have had the struts softened a couple of time since, and I've had one front and one rear seal failure (requiring rebuild again). The failure of the rears is no surprise being from a rally car.
I may be getting old but I've had enough of the very stiff "Targa spec" suspension set up I had. At low speed movement (ie. driving down motorway) the car was harsh as the shocks were immediately stiff. I've now had them softened but they are yet to be installed.
Japanese shocks have a reputation for being stiff and harsh. Only one way to really evaluate them though.
I went for adjustables because when I did this (6-7 years ago) ST165 springs were hard to find (understatement!). Race springs are easy to get in different stiffnesses and lengths to allow for changes.
I've lowered my car and 30-40mm, intend to go no more. Looks great with 235-40ZR17 tyres.
Also be aware that front camber plates makes the car dramatically stiffer. I had the car all set up nice and then went to camber plates. Had to go stiffer to compensate, mutter grumble...
Camber plates for the rear are not necessary. The stock top mounts are fairly solid and Toyota do camber bolts to allow for better adjustment.
I never needed swaybars on the ST165 as the handling was fine with stock. I'd be interested what is avaialable for ST205 in NZ and at what cost.