Wagon suspension upgrade

[Benlooker]

Hi all, new to the forum and to Subarus. Just purchased an 04 wagon complete with the ppp. 

I apologise if this question has been asked a million times before but when I search for wagon suspension upgrade it shows every post with every word in!

I'm looking at upgrading the rear arb, i believe the wagon is 17mm to the 20mm on the saloon. Is this just a straight swap over? Also is it best to use a genuine Subaru arb or will a pattern part be just as good? Or should I use an aftermarket one?

Whilst I'm under there are there any bushes that you would recommend changing? 

Thanks. 

[mermade]

theres plenty of bushes available on the back end, all worth an upgrade since theyre fit and forget https://powerflexstore.co.uk/subaru/impreza-turbo-inc-wrx-sti-gd-gg-2000-2007/