Thread resurrection time.
I am currently having the same dilemma, the Y pipe has failed on my Z (roadster) and was looking for a system anyway.
I have been offered a Cobra (resonated) with y pipe for £700 and a Milltek for £800, wanted to keep the price down but also want a quality system, the car isn't used as a daily driver, more of a weekend toy so was looking at all options.
There seems to be a bad report on the Japspeed K1 & K2 systems due to poor fitment but can't find any info on the K4 (HKS style) by anyone that has actually fitted one.
Also looked at the ARK and invidia Gemini but to be honest I would prefer somthing UK based in case any warranty claims are required.
I like the Milltek as I have used their systems before (on a subaru) and found the fit to be fantastic, The other advantage with it is it can also be supplied and fitted by my local performance store.
I also know the cobra systems are good quality and it is cheaper.
Not sure if I want to go for HFC's or decat in the future though, with the Milltek that is still an option.