the cooling of exhaust gas post turbo will effect the exhaust over all efficiency due to as silver said denser gases. the cooler the gas the denser , the more force it takes to move the less efficient the flow. that being said the laws of turbos show that you want good velocity accross the turbine to allow more gas to mechanical work exchange. if it flows to fast you will loose exchanged work and if it goes to slow you will not build boost. these is a fine line. also there are two forces at work, volume and velocity, if you maintain pipe size you will increase one with the other but if you increase pipe size you can "move" more volume but you lower your velocity. the old finger over the hose trick. when you cover half the end of the hose with your finger you are increasing velocity but the volume doesnt change because the hose valve is the same. increasing exhaust pipe after the turbo is fine line. it would be better to remove restriction to help performance before increasing pipe size. imho