I've done similar and the game played out much better with magic items being more important but absent actual support from wotc it falls prey to any single player at the table deciding "there is no way this can work or be fun" and playing in a way that ensures the decision becomes a self fulfilling prophecy. I'm not going to do the math on your point buy but would suggest a 6 & 4 at +1 & +2 (just skip 5 & 7 totally). since that widens the gap between minmaxed primary attrib & dump stat to a meaningful degree in play and still rewards players who want a more rounded midrange attribute array suitable for broader &less focused goals.5.5 is criticized as being the most powerful edition of the game so far. Which might be challenge if you want to play in something like a Ravenloft campaign, where unbridled heroic power might limit the options for interesting encounters. If you are looking for something lower power which doesn’t limit your characters ability to progress, how does the following sound as an option?
Much of heroic power is tied to ability scores - HP, damage dealt, saving throws, spell DC etc. While at the same time multi-ability score dependency has become less prevalent with the many subclasses and spells that grant primary ability scores to damage and to hit.
You could just reduce the points buy but another option is to increase the cost of higher points. Keep your 27 point buy but have stats start at 7 instead of eight and increase the scaling.
8 - 1 point
9 - 2 points
10 - 3 points
11 - 5 points
12 - 7 points
13 - 9 points
14 - 11 points
This will give you and average of 10-11 and allow people to push their stats to a maximum of 16 (with background bonus) but only if they trade in some significant weakness elsewhere.
I guess the question is what would the consequences of this be? Would everyone just drop Charisma and Intelligence and you’d end up a party of Neanderthals. Or would niche protection become a bigger thing as players specialize in one area alone?
Would having a primary ability score two points lower put you off playing the game, if everyone else is in the same boat? For the purposes of the discussion if you’re looking for a heroic game then obviously this isn’t going to be something you would want, so that kinda goes without saying.
That's why I don't consider any stat to be a "dump stat." All of them are important to every one of my PCs. That doesn't mean that some can't be low or I won't roleplay a low stat, but I don't believe in making a stat low to dump it there because it's not important.
Roleplaying dump stats or horrible 3d6 rolls can be funny for one-shots, but it sounds exhausting when doing anything long-form. At some point you gotta ask yourself, is Ravenloft about People braving the Dark and Triumphing over Evil TM or Grog the Barbarian being really really deficient in crystalline intelligence tasks.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.