First can I assume you have a new age WRX? Sedan or Wagon?
When you say aftermarket STi suspension, do you mean the struts and springs from the new age STi sedan? These are the set available from SOA or are they the UK takeoffs or the Japan 4-way adjustables?
Most people will probably tell you that the STi suspension are too expensive, not worth the money.
Depends what you want, what you are using it for and how much money you have. Most will also tell you to get coilovers for more drop.
As far as the harder bushings, they will help with the handling. The down side is the cost of installing them.
Give some more details. I'll let you know what suspension mods I have later.
The kit you are refering to is the SPT (STi) sold by SOA (your dealer). This is a very well sorted kit and also carries a hefty price tag. The entire kit consists of a smaller (19mm vs 20mm) sway bar, aluminum front control arms, stuts, springs (~200 lbs front/ 180 rear) and some (not all) Group N bushings for the rear suspension.
In addition to the rather hefty price, installation of the bushings adds a considerable cost (several different bushing tools are needed).
Myself, I would forego everything but the struts/springs, but add topmounts (STi). AVSport sells STi strut/spring takeoffs (Europe) including topmounts for ~ $1250. Later if you decide you need aluminum arms, firmer bushings in the rear- you can but these seperately.
For someone wanting a well sorted suspension and doesn't need/want adjustment, the STi strut/spring combo is tough to beat.