I just tried this 2 ways:
first in the init.sqs
? benchmark > 2000: setviewdistance 2000
? benchmark > 2000: setterraingrid 6.25
and it worked like that, but when I setterraingrid 3.125 my game crashed, my benchmark is 3333
also you can make triggers
axis a 0
axis b 0
activation none
condition benchmark > 2000
on activation setviewdistance 2000
axis a 0
axis b 0
activation none
condition benchmark > 2000
on activation setterraingrid 6.25
jst be careful where you set the benchmark, because even though I was able to load viewdistance 2000, terraingrid 6.25, my framerates were BAD!
I read somewhere the default viewdistance for MP is 750, don't know if there is a default terraingrid.
Hope this helps