well i hope I dont come across as someone who needs to take "the cheapest option"
I just took advice from someone who i thought may have been a little more experienced than any random person on a internet page...
also did a little bit of research on the internet aswell
yes 3 inch is probably better but considering I wanted a undertank system and dont particularly want it to be dragging on the ground all the time 2.5 fits nice and i very much doubt there will be much restriction on the flow considering how straight my system is