Buses that are on off service can use the tunnel 
HGDEP off service buses will sure use the KPE one, since the
depot is along the area. One example is services 159 and 163
off service between HGDEP and Toa Payoh interchange
.
Maybe those Premium ann CityShoppers' services along Pasir Ris /
Punggol areas should be diverted to use KPE instead of jamming up
PIE / CTE.