Here’s some numbers (spark requirements) for a high spark craft, which I collected manually.
0 coils 20000
1 coils 15323
2 coils 13938
3 coils 12809
4 coils 11902
5 coils 11187
6 coils 10633
7 coils 10216
8 coils 9909
9 coils 9693
10 coils 9546
11 coils 9452
12 coils 9397
13 coils 9368
14 coils 9355
15 coils 9351
16 coils 9350
17 coils 9350
18 coils 9350
19 coils 9350
20 coils 9350
21 coils 9350
22 coils 9350
23 coils 9350
24 coils 9350
Presumably the curve is similar for crafting time, and flattens to negligence around 16 coils over the requirement. But if you are looking at an item that doesn’t take long to craft, it’s going to be hard to see any benefit from those last few coils.