John - Do you come in each lap and corner weight based upon the fuel used so far? No.
Now take that compromise and expand it a bit, I'm not saying the car will be perfect but then no car is once it has used some fuel etc. The fuel load in my car (and indeed your M3) could vary by 85-90KG if it started with a full tank and ran to the reserve collector then add some driver weight varience on driver change, still worth doing though IMO and I assume you'll still corner weight your e92 for British GT?
You found 0.7 seconds a lap through corner weighting your vx220, if you could find half of that by being half way towards perfect from your starting point then it is worth doing no?
Of course it isn't going to be perfect at all points during the day but then geo won't be either after you've banged it off a load of curbs and people still set that.
Anyway, my opinion is just that, not trying to change your mind just putting another opinion there for people to read, I've not run a race team so that puts me on the back foot anyway. No point in arguing
