So upon doing the math, the best idea I have so far is this:

+25% sapping speed
-25% ammo/metal from destroyed buildings

-50% sapper health (1 wrench swing/2 jag swings)
Doesn't disable buildings

I found out what the sapping times are and did the math

Normal sapper
-Level 1 building: 6 seconds
-Level 2 building: 7.20 secs
-Level 3 building: 8.64 secs
EDIT: - mini sentry: 4 secs

Proposed +25% shredder speed
-Level 1: 4.5 secs
-Level 2: 5.4 secs
-Level 3: 6.48 secs
EDIT: - mini sentry: 3 secs

Considering how low the sap times already were and making sure they didn't just Insta sap a building to death 25 sounds way fairer than the other threads proposed 50% speed increase.
Sapping a level 3 with these stats is almost as fast as sapping a level 1 with stock and sapping a level 1 with these stats is almost as fast as sapping a mini sentry.
So it still benefits the spy while also remaining within fair numbers for the engineer, and even still the engi can just whack it away in one hit with their wrench and don't have to worry about getting sapstabbed when a spy is using the shredder because if it's on the sentry the shredder can kill it but can't disable it.