Spin - I've consulted someone who has a way better understanding of all things mechanical than me about the risk you were told about just running the DP and Cat.
Here's what he has to say:
"Freeing up the back pressure in the downpipe by increasing the flow, going to 3" from 2.5" and from a restrictive 400+ cel cat to a HJS 200 cel race cat will lower velocity and decrease restriction and therefore temperature in the exhaust (Exhaust Gas Temperatures EGT's)
So, less velocity and less heat will be better for the cat back section of the exhaust.. Thats my take on it anyways? Unless they mean that cooler exhaust gas flowing more slowly will have an adverse affect on things?? lol.. "
No doubt a full system is going to be better... but the above statement kind of makes sense to me!
Who told you it would cause temp issues if you don't ind me asking?
J