Left front catalytic converter sputters. What is the most likely reason in my case?

I have 2 Mark VIIIs, one of which my brother has been using to move around. I have always used 93 gas on the Mark he's using before giving it to him and it ran perfect, but he goes for the cheaper stuff and always lets the tank go damn near empty before refilling. Four times it ran out of gas on him and after the last time, he's been reporting random slight shakes when sitting idle, slow pickup after a momentary loss of RPM when throttling from a complete stop, reduced MPG as well as reduced hp & tq, but no Check Engine light.

I got to the car and cleaned the MAF sensor, checked the spark plugs & COPs, replaced the air & fuel filters, but I have not tested the injectors individually nor the vacuum & fuel pressure and have a can of Seafoam ready to use.

There is also a noise coming from underneath the front driver's side of the car, kind of like a struggling exhaust as it goes "puh puh puh puh puh". I move to the passenger side near the front wheel and I hear it slightly, move back to the driver's front wheel and it is much more distinct. I can only guess the left front catalytic converter is struggling due to my brother's choice of gas from mine and his low fuel maintenance having affected the converter or perhaps it something else completely unrelated from what I'm assuming.