I agree with Boris, you must be doing something very wrong. Are you actually
driving the cars, you know, just by feel? Without thinking about it? The fastest laps happen when you're "in the zone", meaning it's totally your unconscious doing the driving with no active thoughts about brake points or similar whatsoever.
Over 6000 laps is a MASSIVE amount and the changes on Blackwood aren't that dramatic. You should be way better than 4-5 secs off WR. Heck I just did my first ever 4 laps on FOX/BL1 since patch Y (LFSW says I have 293 laps done on it, but they were all done a long time ago), and I managed to get a shitty 1:10.72 with the RACE_S setup and a not properly mounted wheel, and that time is already less than three secs off the WR. Okay, that doesn't help you in the slightest, but seriously, you must be doing something
fundamentally wrong. I'd suggest you to practice more, but maybe you actually need to focus on the way
you drive
You also seem to be overly obsessed about setups, whereas their relevance to race speed isn't that high. Sure, the setup needs to roughly fit the track, but the fine tuning of exact values is for when you get into the sub one-second-off-WR range. I usually just take a setup from the setupfield (preferably the ones that come without a locked diff), do some blind tweaks and fixes to it without even testing it beforehand and most of the time it's perfectly fine for racing right off the bat. Completely developing a own set is just too much effort for the amount of racing I do these days.
Though granted, it all also depends on how gifted you are. The people you see doing WRs left and right don't necessarily practice months on that track, on the contrary, I'm pretty sure most are up to asskicking-pace within a few laps on a combo they've never driven before. Guess some people just have a knack for "interfacing" with computer sims or games in general