The shadow Spectre by itself are already incredible OP but giving them an additional -1 to hit plus stratagems will be absolutely devastating.
Deepstriking stratagem together with the autarch for hit rerolls or farseer guide plus warlocks additional -1 to hit or +1 armour save. You could buff them up to -4 to hit if you want to.
If you don't know them :
M12, BS3, WS3, S3, T3, W1, A1, Ld8, 3+save
-1 to hit by default
Craftworld elite aspect warroirs
Each one has a weapon with 2 profiles:
18" assault 1 S6 AP-3 D1 (every hit generates an additional hit until max 3 hits per shot)
8" assault D6 S5 AP-1 Auto-hit
That means if you can't shot them to death you are looking at potential 10 heavy flamers in one unit of 10 during overwatch.
23 points per model. I don't think you can't find any infantry that powerful in the indexes or codexes with that kind of synergy.
FW is really breaking the game with such units.