As others have said, the speculum swaps BASE stats. It doesn't take into account magic or mundane items, augment or hex spells, or any other special rule such as frenzy that would add to attacks.
Consider: Arch Lector is fighting Templar Grand Master.
Arch Lector: Ws4 s4 I4 a2 s4
Templar
GM: Ws6 S4 I6 a4 s4
Let's say, for fun, that the arch lector has the fencer's blades and the templar
GM has the sword of bloodshed.
Now, if the arch lector has got the fencer's blades and he chooses to swap stats, what happens is that the arch lector's
WS will go up to 6, which won't matter at all because his weapon is making it 10 anyway. However, the templar
GM will go down to weapon skill 4, meaning he'll need 5's to hit the WS10 pope.
Conversely, the pope's attacks will go up to 4. He will get an extra one from the fencer's blades for a total of 5. The templar
GM attacks will go down to 2, but his magic weapon will buff him back up to 5 attacks as well.
In short, when stat-swapped with a speculum, follow these steps.
1) Take your opponent's statline (
WS, S, A, T, I)
2) Add any bonuses from items, special rules, or spells that YOU are carrying.
3) Use the results in resolving attacks.
edit: as you may be able to tell, I play empire in a club full of warriors of chaos. Speculum is mandatory kit.