bigger is almost never better
Your collector is and had better be a tuned length and diameter for your engines needs. A three and one half inch conventional collector is real big on 700HP FYI. If you run a exhaust pipe the same size as your collector the engine see's a 8' long collector and tunes the power band for 2000 RPM or a lot less RPM if you will, the longer the collector the lower the tuned length. For reference, your engine can have a collector length between a short 10" in drag only trim and long 24" in street trim maximum. You can use a reducer cone at the end or your collector that reduces the size of the pipe a 1/2" or so so the velocity of the gas changes to tell the engine where the collector ends, then you can go back to the collector size if you want to. However, you will not see and advantage with the huge diameter collector pipe extension. The pipe weighs a lot and bends get very expensive or restrictive and you loose heat that in turn hurts gas velocity.
Bruce