Nonsense! Your maximum becomes 3 lower, but your actual SP increases by 3/6 versus pure Sorcerer (1 lower max, 2/4 higher compared to Warlock 2).
Warlock 3 gives you level 2 * 2 slots * 3 rests = 12 SP.
Sorcerer 20 gives you 3 more base, and an extra 5th, 6th, 7th slot = 21 SP. Plus the capstone for aother 12.
But I guess we are talking warlock 2 vs 3. So that's 1 extra level * 2 slots * 3 rests = 6SP.
Sorcerer 18 gives 1 more base, and 5th level slot = 6 SP.
Net is the same SP either way.
So it's down to spell bombardment (+0.55 damage per EB ray) vs some THP.
I'm... not sure where Greater Invisibility comes into it though? Unless you're on about the familiar.
For advantage. EB spam greatly benifits from it, and sorcerer's don't get foresight. Twinned Greater Invisibility helps both you, and an ally fighter/rogue/paladin/monk/ect...
Darkness + devils sight would be cheaper, but only work for you. It also requires warlock 3.
Generally, I favor party total over personal. Particularly since you're hiding behind them.