Random starting problem - fuel related

[blocke]

Unless undersized, a new catalytic converter would improve fuel mileage as it would have less restriction on the exhaust flow.

Leaking or dirty injectors will decrease gas mileage. But there are other possible causes. Like intake leaks, incorrect rail pressure, a bad ECT, a bad AFM, bad ignition wires/plugs, misadjusted valves, a cold running engine, or an aged O2 sensor.

Yeah brand new CA legal magnaflow cat. I'm sure its the correct size for the car. Should be more free-flowing.
ECT? Jog my memory....?
Intake leaks are possible ( have yet to get smoke test done)
Few thousand miles old Miller MAF
Haven't checked rail pressure
Stock thermostat
valves were adjusted recently ( i suppose if eccentrics are fubar they could be out again)
02 sensor is less than a few thousand miles old
brand new plugs/wires/coil/cap/rotor