i think only shimano knows for sure, and even they might not be certain.
so the deal is this.... the shimano tld 30 II frame carries a risk of failure at drag settings in excess of 20#'s at strike. the tld 20 II frame should actually be STRONGER because the frame is narrower. there is less base between the two side plates. the aftermarket tib or willfish frame totally eliminates this problem.
so what is the next weak link?
it's the anti-reverse dog. it is screwed into the graphite side plate. i have never seen it happen, but having the dog ripped out of the side plate is the next logical place for the reel to fail. i have no idea if this is even possible.
i like the tld 20 two speed, even with the graphite frame. it's a great 4/0-sized reel. i can rebuild mime in my sleep (and sometimes do). they are loaded with 80# spectra and a topshot of 50# mono with a drag setting of 30-40%. i'm not a big fan of the wide spool tld 30 II. i think it's too much spool and not enough side plate. either way, you still have to deal with a graphite side plate. so for nearly the same price as graphite, i would rather see you get an old avet ex 4/02.