HKS F-Con is without doubt one of the best on the market. You will need an HKS powerwriter dealer to set it up and to be honest TDI have by far the most experience speak to Mark Catchpole. If you want to go the UTEC route then Thor Racing are UTEC dealers speak to Pete Betts.
Id recommend Tony at GT Export, hes been tuning Z since they were on the market, very knowledgable and sorts out mine, mums and dads, car he use to work for TDI then went his own way, been tuning for 15 years and done my brothers car and tuned my sisters, he also does the full works and gets it on the rolling road too, has a really good workshop in Essex with loads of Top Jap cars and BMW tuned in his garage. Me and my brother swear by him He's been going to a lot of meetings lately where they are working and understanding prodrive tuning for the imprezas, he knows 350z inside out, let me know if interested ill email you his number.. if he has one fault its that he is a perfectionist and will chew your ear off on the mechanics of tuning cars given half the chance. he is at his garage from 7 in the morning till sometimes 12 at night He lives cars, he had a 500bhp heinmann tuned bmw m5 unbelievable