Welcome to EBPF, sorry to hear your vehicle is having issues!
You are right, exhaust leak would be a good suspect. Sometimes you need to install new gaskets, even doubling up on them to make sure the seal is good & tight.
O2 wires often get nicked in the process, and then the sensor has to be replaced.
What brand of plugs are you using? Did you use new plugs or re-gap the old ones? If old, how many miles on them?
Ford has a TSB for concurrent p0420 and p0430 codes, but if you do choose to recalibrate, return your SHO to stock before having it done. Then get the new strategy code and have your tuner update the tune accordingly before re-installing on the SHO.
http://www.ecoboostperformanceforum.com/index.php/topic,3836.msg79035.html#msg79035
May need wheel hubs replaced due to that C1B00 code. See more details here:
http://www.ecoboostperformanceforum.com/index.php/topic,3965.msg62365.html#msg62365