Misfires: Any number of things, possibly lightened flywheel. Check ignition stuff and check the flywheel model to see if others have had misfires with it.
P0136 and P0245: Could be due to downpipe. If present, the sensor could be faulty. The latter could be a boost issue too though - are you creeping?
P0546: EGT sensor problem in the up pipe. If the correct pipe is fitted, check the connections. If the pipe has been replaced, use a M12 bolt to plug the hole and use a 2.2 kΩ resistor in the harness to prevent another CEL.