Ok, I found the source of my problems. Firstly, I moved the brake bias forwards a couple of clicks to help with locking the rear under heavy braking. Somewhere around 52.5% IIRC. Although I felt more in control under braking, it didn't really help laptimes as I was still struggling to hit consistent 2:28s. I was really frustrated by this point and about to give up for the night when I thought I'd see if there were any hosted sessions using this car which I could join and spectate the fast guys. There was, so I jumped in.
The only differences between this session and my offline testing were the weather (this was cooler, lower temperature, higher humidity and more wind) and that this was the endurance pits, not GP pits which I had been using. My first flying lap was a 2:26.0... WTF? Instantly 2.5 seconds quicker. Ended the session quickest with a time of 2:25.0, and an optimal of a 24.1, easily running consistent laps between 25.5-26.2.
I'm really confused. Obviously weather makes a difference, but nearly 4 seconds? This was the exact same car, with the exact same setup. Does the track rubber in with more cars? There were about 12 other cars, a mix of HPDs, FGTs and other C-Specs. Is there different grip levels in the different pit layouts of Spa?
I'm happy to at least have something resembling a competetive time, but I'd really like to understand the difference between these sessions.