Digging into the data a bit to see where and how the 1.5 seconds was gained when compared to last year.
So again, here are the changes to the car when compared to last years WGI race:
10" vs 9" wide wheel
RT615K vs RT615K+
no synchro issues with new transmission
3.69 vs 4.08 FD
stiffer spring rates with no rear bar (600/500 vs 500/335)
MCS 2 way remote dampers vs Feal 441
Below is an image from AiM analysis software (RaceStudio) showing my personal best laps from last year (2:10.6xx {red line} and this year 2:09.3.xx {blue line}). My co-driver is the one who usually sets the fastest laps, but unfortunately I don't have his data from last years 2:10.0xx to compare to his 2:08.5xx. But, it's basically the same difference between our bests; 1.3 seconds for me, 1.5 for him.
So, 80% of the time gained this year from last year is from the final drive ratio and butter smooth new transmission. The bottom most row line graph is the lap time delta. You can see from start/finish all the way up to the entry to turn 4 ("bus stop") the line just keeps climbing steadily. I don't have the TPS shown there, but from the exit of turn 1 up to turn 4 is full throttle the whole way and it's significantly uphill through very high speed turns, called "the esses." A good ball tester.
Look how brutal the 5-6 gear shift from last year was on the speed graph in between turns 2 and 3
With that said, lateral accelerations and speeds through the turns are basically the same on a less sticky tire this year. Perhaps a testament to the new dampers and spring rates.