I think the stock system has some inherit flaws regardless of E, maybe E exacerbates it, maybe not.Answer is, 'safe' levels is the ethanol included from the pump when using regular octane gas. Any e blending and YMMV. I've seen people run e30 on stock pumps no problem, I've seen people install the HPFP and blow an injector on 93 before they could even add E. I've also seen an injector blow on stock pumps, 93 only. No 2 cases are similar. The moment you add E, you are at an inherent risk of something failing.
"The moment you drive off the lot, you are at an inherent risk of something failing" revised it for you.