One of the things I liked about AD&D and BECMI was rolling ability scores. Was pondering recently how it could be adapted to 5e and "levelling" beginning characters with feats. e.g. mid range = one feat, lucky rolling = zero feats, unlucky rolling = two feats
So rating would be determining the overall "value" of the dice rolled, maybe according to this table - question is - what point spread = "normal", and where are the bars to one more or one less feat (or more tweaks)
But the default in Fifth Edition is rolling ability scores randomly, using the old best 3 of 4d6, rolled six times then assigned to ability.
Giving them a bonus feat for low stats might be unbalancing for certain characters, particularly those with an excellent number in their prime stat and mediocre or low ones in others. Low ability scores shouldn't be worth a lot of "compensation" or the system's too open to min-maxing. Consider, say an Intelligence 18 Wizard with 3s in Strength and Charisma. Being a hideous weakling doesn't make their magic any less powerful.
The "negative ability value" you've proposed for ability scores below 8 seem too high to me.
Using the proposed numbers, a character with the ability array 3, 3, 14, 16, 16, 18 is a 26 point character. Is such a character really slightly weaker than a regular point buy character?
If I had a starting PC with overly low abilities in my game, if I wanted to make their life easier I'd probably give them something like bonus equipment rather than adding something integral to their character like an extra feat.
To balance high or low ability scores, how about doing something with Ability Advancement? Maybe PCs with low stats advance faster. Perhaps they get +1 to three stats or +2 to one stat and +1 to a second stat until their stats stop being "low", however that's defined.
Contrariwise, perhaps a PC with high stats either gets a poorer Ability Advancement (+1 to one stat instead of the +2 to one or +1 to two that's normal) or they're only able to apply their Advancement to their weaker stats, so a character with 8, 10, 12, 17, 17, 18 that hits 4th level can only apply its two points of Ability Advancement to it 8, 10 or 12 abilities, rather than putting +1 on each of the 17s and getting three 18s.