I've been thinking of getting some different engine management.

But I don't have too many goodies away from stock:
Blitz intake filter
Cusco catless downpipe and FGK cat-back....may replace with a catless uppipe soon.

that's it and i don't intend to go any further, and i just want to take full advantages of these as a whole.

Should I have my stock ECU reflashed? or should I purchase a seperate module? What are the comparisons of the two options?

 

Cobb Tuning reflash

 

agreed

 

Rallispec.com also has reflash service, does anyone know the comparisons?

And, since I don't live in UT or NJ, is there similar service provided in CA? I am concerned in locations since I always have the feeling that I may need to ship the module back if there's anything wrong.

 

Living in CA you have a TON of places to go to. One of the mods of this board, skywalker(Bill) can tune just about anyting you have on your car and he works for i-speed based in northern CA I think. i-speed is a reflash dealer ala ecutek. He is also a master tuner when it comes to LINK tuning.
www.i-speedusa.com

But you also have vishnu tuning and Shiv is quite renowned for subaru tuning. He can tune reflash your factory computer, sell you a Xede which is a piggy back ssytem, or go wild and crazy with a TEC III system for ultimate HP and absolute control of your engine and it's parameters. They are in teh bay area I believe.
www.vishnutuning.com