Just an idea: perhaps MAF is reading too low thus causing load (rl, rl_w) to read low so it picks a timing value from a too low load column (lower engine load = more ignition advance usually).
Compare ps with actual boost pressure.
The actual load reported by vcds is in the mid to high 90s. Throttle is 99% open, the timing though is around 16 to 17 at 2100 rpm. About 6 to 7 more than the maps I modified.