But burned all 5 of your sorcery points....
EDIT: to elaborate, both versions cast darkness two times, yours with 1st level spell slots, the UA version with sorcery points, then we can “rebalance” two different ways: either by you turning points into slots, in which case they end up with same remaining resources and are equivalent, OR by the UA version turning slots into points...in which case he ends up using all four of his 1st level slots, or two of his second level slots, whereas you’ve only used 2 first level slots. It’s the asymmetry of the conversion that gives your version a big advantage.
I think we are losing each other here. So, let me put it down and see if I can get it.
At 5th level you have 4 first, 3 second and 2 third level spells. Along with 5 sorcery points.
In the current version of the shadow sorcerer, if you wanted to cast a bunch of darkness for minimal cost, you could cast it three times by converting a single first level spell. So, no sorcery points, 3 first level spells, 3 second level spells and 2 third level spells left.
In my version, you would cast it four times, using all of your first level slots. So you would have 5 sorcery point, 3 second level spells, and 2 third level spells.
So, my version got a single casting of darkness more. But is completely out of first level spells. Both versions have not used and second or third level spells, so we should compare the 5 sorcery points to the 3 first level spells.
And the assymmetry makes this hard. If we take the 3 first level slots and convert them to points. Then the base version got 1 less casting and only have 3 points instead of 5. But, if we take the 5 points and convert them into first level slots, then my version ends up down a first level spell (they can only make 2) and has a single sorcery point which depending on metamagic choice might be worthless. So, worst case is an extra casting of darkness for the cost of a first level spell slot.
As a note, I would point out that while Darkness is a 2nd level spell, in terms of what it actually does, it is directly comparable to Fog Cloud, a 1st level spell. The big differences being that Fog Cloud can be blown away by wind and creatures (devil's specifically) can see through the darkness spell.
So, if there is a substantial increase in power, I would say it only shows up in the fact that you can cast Darkness and still have access to your sorcery points. The very thing that makes the sorcerer the sorcerer. But, how powerful that is depends on how valuable you see sorcery points compared to first level spells. I think the spells tend to be more valuable, because the points can't be meaningfully spent without spells to spend them on.