i have bought an eu uk spec imort impreza wrx what alarm will i need does the car already have an cat 2 immobiliser so do i need a cat 2 to 1 upgrade

Decision is yours really, see what your insurance co require. More than likely they'll insist on full Cat 1, not Cat 2:1 (upgraded 2) and a tracking device.
Only benefit in not having a tracker is, if it's stolen when you get it back it's worth nowt 2nd hand as a stolen/recovered, so many don't want to see them again.

I think many companies accept a 2 to 1. I know Direct Line do for one. Obviously if you've fallen victim to joyriders it's a different story, but most professional thieves will take the vehicle off to a quiet road and watch it for a day or two to see if it gets recovered, so a tracker may not be such a bad thing.

Hate to say this, but the 03 standard install is fitted more like a 2 to 1 upgrade...