I know I shouldn't argue with you about this, but I thought on low speed sections the grip is so low that you can't put all the high power down, so engine doesn't matter so much. And on the contrary, when you are accelerating from 100 km/h upwards you rarely have problems with the grip and the you gain seconds by hitting your top speed earlier than other cars. I don't know, but Suninen was up to pace last year in Sardegna, while all the M-Sport drivers were horribly lost in Estonia...
We see this also when we compare the pace of R5 cars, on fast stages the WRC cars are much faster but on a technical stage the R5 can even get in the top 10 (or even win the stage like Huttunen in Sweden 2018)
I would also say Greensmith has improved his driving, hence him getting closer to Suninen. The stages are also somewhat simpler, not so many corners where you can make a difference.