I'm surprised by the change from a Qualcomm SoC to MediaTek. The Qualcomm Snapdragon 430 in the old Nokia 5 was a highly significant factor in my choice to buy it; and I've been very satisfied with that choice. If the Nokia 5.1 had been based on the Snapdragon 450, it would have been an easy choice of direct refresh-upgrade for me. This change, to what I regard as an inferior SoC vendor, makes it a difficult choice, and I think I may well either delay or look elsewhere as a direct result.
What is your source for those performance claims? 40% faster than the SD 430 seems possible, but I have significant difficulty seeing how 27% over the SD 450 could even remotely be possible. Everything I've seen so far suggests to me that the P18 and SD 450 are quite close in performance.