304 SS is fine to use, provided the welder uses proper welding techniques. 409 has a higher iron content which makes it tougher, but it corrodes more than 304. However, 304 can crack over time, especially at the welds.
But otherwise, the primary cats are more restrictive than the secondaries. I'd rather remove the stock DPs than take the time/money to remove secondary cats as far as power upgrades go.
E92 335i/6MT Dinan Stage 3: intercooler, oil cooler, intake, flash, exhaust, LW dual-mass flywheel.
Vishnu PWM methanol custom trunk mount, Diffsonline 3-clutch Salisbury custom diff, Ohlins Road and Track coilovers, Vorshlag camber plates, much more...
E28 M5 w/ buncha stuff, R171 SLK350