Airbag light should not go on/off. Once it trips it should stay illuminated until [code read and] reset by Durametric, PSTII or PIWIS - as I understand how the systems work. Might it have been a CEL which is probable considering O2 sensors and cat bypass (which require a spacer correct?)?
The 3000 studder sounds like relearning of ECU. Should be fine w/ ~ 50 miles or so.
Whistling definitely points to a leak somewhere in system. A quick test, if you have some handy, is to put some exhaust heat wrap on the connection. Though it feels tight by hand, once under load the whole system flexes a fair amount and at 4000 you're getting into the power band.
Good luck
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