TL;DR -- I think the large greensteel shield needs to have its CP/DC reexamined, as it no longer serves the purpose it once did.
So it occurs to me that the greensteel large shield is in a strange place. Let me explain:
Adamantine Large Shield costs:
1x adamantine ingot
1x wooden large shield.
It has a DC of 32 and costs 192 CP to make.
-It has a +3 shield AC bonus, and 10% piercing immunity. It's not a bad shield!
The Greensteel Large Shield costs:
4x Greensteel Ingots
2x Coal
It has a DC of 51 and costs 500 CP to make.
It has a +3 shield AC bonus, 10% ASF reduction, and 10% positive energy immunity, positioning it well for builds that want to equip a shield that get at least 5% ASF reduction naturally.
Based on this alone, you'd think, sure, okay. The greensteel large shield clearly provides additional benefits over the adamantine shield and has the same shield AC bonus, so clearly, it makes sense that it is much more difficult to make, as greensteel is significantly easier than adamantine to obtain, and significantly cheaper. The costs balance out!
But here's the thing.
The enchanted shield -- a large shield -- provides a +4 shield AC bonus, and can easily take a lesser etched rune, allowing you to customize it easily. It's probably the -best- shield for the price right now, and if you don't need the piercing immunity, and just want the AC, it's probably the -best- large shield. It contains adamantine chunks, and is a pain to make a lot of unless you prep a lot of materials. Despite this, its superior AC largely invalidates the use of the greensteel large shield for people who might want to use it for other reasons.
The fencing buckler -- a small shield -- provides a +4 shield AC bonus, and being a small shield, has an ASF of 5%, which is negated as long as you have at least 5% ASF reduction. It's on par with the greensteel shield so far as AC goes, and can easily take an etched rune. It has a DC of 37, and costs 280 CP, making it a cheaper, easier to produce option for spellcasters who may want the greensteel large shield for the AC bonus it provides. It takes one brass ingot, one mithral ingot, and four coal to produce.
If I want AC, and I -need- to use a large shield, I'd go for an enchanted shield and skip the greensteel, as it requires a master smith, and is -ridiculously- time consuming to make, inflating its total price considerably. It's not worth taking that step. If I want AC, and I need something that works well with the amount of ASF reduction I have, I'd probably go with the fencing buckler, as it provides the same total AC for less resources and time.
Which... leaves us in a weird place. See. I don't think that 10% positive energy immunity is useless. But is -this- the single, solitary feature that necessitates the large greensteel shield having its ridiculous crafting point cost and DC? It works in very, very niche circumstances in the current meta right now, and not very well.
I think having its CP and crafting DC reduced a bit wouldn't be a bad idea.