Mystical Lightfighters

This was confirmed by Ngreth:

For raid gear:
10% for 1 piece. (aka, only bp)
20% for 2 pieces.
30% for 3 pieces.
40% for 4 pieces.
50% for 5 pieces.
60% for 6 pieces.
70% for 7 pieces.
100% for 8 pieces.

Group gear follows similar structure, except its max is 70%.
7% for 1 piece( BP)
14% for 2 pieces.
21% for 3 pieces.
28% for 4 pieces.
35% for 5 pieces.
42% for 6 pieces.
49% for 7 pieces.
70% for 8 pieces.

How does it work if you mix and match raid and group gear and gear of different tiers?

From my observations group gear must match the tier of the BP to count.

For raid gear, the Tier does not matter, at least on SoF. If you have an MMM BP with Crystallos in every other slot you will get max stats, they all count.