just wonder what is the main difference btw the test pipe and the full catback?

 

You are replacing the catalytic converter with a straight piece of pipe (thus, test pipe). It has little to do with a cat back system. You can have a test pipe or no test pipe (retain the cat) with a cat back system. The only trouble is that you need to find an electronic device to fool the ECU since the O2 sensor will not be reading anything or it will be reading the wrong thing if plugged into the test pipe. It you don't, you'll get a CEL.

 

what about the performance?? is it help if i just install the test pipe (not the cat back)
also........how much does a O2 sensor cost ??
thx for ur help xviper!!

 

I don't have any first hand experience with a test pipe but if you search "test AND pipe", you'll see lots of threads on the subject. The S2000 already has a very high flowing cat converter, so taking it out not only fails emissions, but may not be adequate gains for the trouble and it's not environmentally responsible.
You don't need to buy an O2 sensor. The cat already has one. It's what to do with the one that was plugged into the cat. I don't know exactly how much the electronic device that fools the ECU costs (50 bucks maybe?) but again, you will turn something up with a search.
On the same note, only a few cat back exhausts will show some measureable gains and most of these are singles and are generally very loud.