if you've ever heard of google you can simply type in 2013 wrx invidia downpipe.
you'll find them on multiple sites for around $510. so $90 or so cheaper than the cobb. cobb build quality is amazing. i have no experience with invidia but ive heard they are just as well built as cobb. the cobb dp necks down to a 2.5" connection at the end so you can hook it up to the stock catback no problem. the invidia is full 3" all the way but you might need a grimspeed adaptor if your thinking of hooking it up to the stock catback. its a $45 part so add that to the invidia price if your price shopping. either choice in DP will net you the same hp if your going OTS map anyways so just buy what you feel more comfortable with.
i dont really understand your question about the percentages? i think cobbs picture of the dyno graph is what confused you. 30% of 244 is going to be less than 30% of 265. dont pay to much attention to their "dyno graph" as it doesnt even have any numbers associated with it