I bought a 3 year old (2014) LML with 29k mi in 2017. Over the first 6 months, I had a few issues w/ my EGR which were extremely inconvenient and annoying. I also had a fuel pump failure at about 70k miles that was covered and replaced under factory warranty. I chalk these up to the lack of consistent run time over the first few years, though I could be wrong. I just crossed 150k miles today. Since about (aboot?) 75k miles, I haven't had a single issue.
FWIW, I would still delete it for all reasons imaginable. If I decide to drive the wheels off this one vs. driving the wheels off a L5P, I'll do some more research and do a full delete to include the fuel system (CP3, lift pump, etc.)
2nd question: I've heard that unless you're racing, you don't need a 5" exhaust setup. I'm confident that 4" would be plenty for the application you intend, while still giving you tons of the desired air flow you and your LML want.
2014 2500 LTZ Z71
35 x 12.50 TA KO2
Otherwise, bone stock