The sizes should be fine. I run 245/35-19 and 275/35-19 on my 981BS, and it's fairly close to stock offsets. You want the rear/front tire diameter ratio to be ~1.02-1.05 to keep PSM happy. This is about ~1.04. OEM 20" is ~1.03.
What tires are you running? Are these for track, AutoX, or just daily-driving? Changing tires can make all the difference in the world in grip, but higher-grip compounds will have quicker tire wear. Also, some tires have a tendency to heat-cycle out and grip will fall off before the tread is gone.