you mean like warlocks are?Pulling numbers out of nowhere helps no-one.
20% buff would make fire no.1 or 2 spec on AOE and ST. Just go through the mythic raid logs and add 20% dmg increase to the average... for instance on star augur fire would be just below affliction(which lets be honest is currently broken af), that really would not be acceptable when most range specs don't excel at multiple dmg roles and would make fire a heavy outlier that can perform way to good on ST and AOE.