To answer your title question: quite. the difference between a level 40 tyranitar vs a blissey between 1 attack IV and 15 is 1,7 damage per second. This might not seem much, but in certain cases its the difference between winning or losing a level 5 raid with 3 or 4 people.
HOWEVER. Just IV doesnt mean everything. You need to check the attack IV. If your 59 chicorita with 98 IV has 14 attack, 15 defence and 15 stamina, and your 93 IV has 15 attack 14 defence and 14 stamina, you are better of with evolving the 93 IV.
But for a Meganium @Necrozmadabest is right. Its not worth putting stardust in. If you really want a good grass pokemon, go with Sceptile.