So, been reading around and changed my exhaust to a dual Invidia exhaust and catted down pipe and what i'm reading is it being said that the up pipe should be replaced also, due to being hard on the turbo so I read. I just had the timing belt changed on my car, along with the water pump and thermostat and pulleys. The mechanic who did the work didn't say much about the up pipe, but his friend is into turbo's and says he can't see the benefit of the up pipe. I bought my stuff from Rally sports direct along with the cobb tuner and air intake. So, what is the benefit of the up pipe and why is it being said to change that as a safety precaution and less as a performance measure?

Your stock pipe isn't too terrible. It's already catless. The only drawback is the corrugated flex section that 'hurts' flow a little bit. Not enough that you would notice. Our early wrx's had a catalytic converter and egt sensor in them that are known to break and be ingested by the turbo.