Is 20 CHA really that important for a sorcerer?

[TheUser]

Spell save at level 8 with 18 CHA = 15
Spell save with 20 CHA = 16

Chance to save with + 0 modifier = 30% for 15 and 25% for 16 (1-14 are failure for the first and 1-15 for the second leaving 6/20 rolls as success for the first and 5/20 for the second).

Here's where it gets weird because a percentage vs another percentage is a new ratio with a new percentage:

30 is 20% more than 25. (30/25 = 1.2)

At level 9 it is now 16 vs 17 for spell DC
So 20 vs 25 percent chance to save at +0

25 is 25% more than 20 (25/20).

Essentially, if you target weak saves the odds of enemies saving is increased by roughly 20-25% when you don't pump charisma.

It's a 5% difference on a d20 but since we're comparing two percentages it's a 25% difference in sucesses for weak saves.

Is that worth a feat to you?