After looking at more logs I think you may be right about the big difference being too optimistic. Where I got the big difference from: this first comparison is the latest setup with V8 E30 and the K&N + extra airfilters compared to the worst log with stock airbox and filter V8 E50 but around the same boost level, 3rd gear:
After this last log I did replace my boost solenoids and vacuum line next to my oil filter because I suspected a leak.
After that I have logs from around the same boost level, also 3rd gear and it does show about 8-10% more wgdc... Maybe that's more realistic.
Conclusion, next test is to take off the small filters and cap off the holes, do some datalogging with the same setup airbox and K&N drop in filter and compare those 2 before making any more statements
