I can agree that these are high level feats and they are supposed to be powerful, but Quicken Spell should be as well, especially since Larian increased SP cost. [...]
To keep power balanced at lower and higher levels between Overchannel / Empowered Evocation and Quickened Spell, Quicken Spell could affect levelled spells only starting from level X, so that it is not abusable at lower levels and have a spell level cap similar to Overchannel spell level cap. It could also have increasing SP cost similar to Twinned Spell (so that Twinned Spell can affect 2 targets for 1 spell slot and Quicken Spell can affect 1 target 2 times, but for 2 spell slots, giving both Metamagic options their niches) to keep it from being too much utilied at higher levels and with higher level spells.
+1 for the increased SP cost. Quickening a more powerful spell in BG3 results in a more powerful turn, so the SP cost should obviously scale with spell level. This is not true in tabletop, because Quickening a spell is mechanically equivalent with Quickening a non-spell (and constant power level) action like Dodge, Dash, Disengage, Dash, or Hide.
I don't think a restriction on levels of spells that Quicken can effect would be necessary with the above cost increase; it'd be a bit redundant.
