I was searching for the article and can't find it. it's somewhere though. anyways, there was a line rating test a few years back that compared various brands of line and the break strength. if I recall, berkeley big game 20lb broke over 35 lbs. in fact, most brands were 30% or higher in break strength. meaning most broke above 27-28lbs. using a good 30# line, you're going to get at least 40# break strength with most lines. the labeling says "30#", but that means it breaks no less than 30. many years ago, without the high tech processes and QC that all now employ, the variations and such meant that some sections would break at 38 and some at 25. thus 30 was more an average. today, lines are far better. I do know for example, I hung a jig a while back using 20lb big game and pulled a helluva lot harder than 20lbs before it broke.
using 30# is going to work for just about any iron application that 40# will and you won't notice a difference. smaller diameter will cast better. at most, you're fishing with 12lbs of drag. and that's probably way too much anyways. no way, with good knots and fresh line 30 or 40 is gonna matter.
BBG is very strong, and I use it on all my heavier stuff. not the best casting line though pretty good, but durable as hell and strong.
flouro is a little different as it's closer to test strength. IGFA rated lines do break at their designation. they have to. spectra is also pretty close to line specs. regular mono is way stronger than labeling for most brands.
damn, wish I could find that article. anyone know it's location???
this might be it, but I think it was originally elsewhere
http://www.sportfishingmag.com/article.jsp?ID=23589