I think people are getting too greedy here. I mean of course a warlock player wants to cast "as many spells as possible", but we can't let warlocks get lots of slots AND have all of those slots be a top tier spell level, that's going to be OP.
If we go with the old 2 SR per day, then a warlock has gone from 4 spell slots to 5, that's a 25% increase in how many spells they cast a day. At 11th we have gone from 6 spell slots to 8, a 33% increase in slots.
To me the only thing I would change is allowing the ritual if you are not out of slots, to make it more user friendly. Otherwise its already a pretty big increase in your overall power per day.