Just because its turbo doesn't magically mean that it is a faster car.
The RB25DE has 700cc more displacement, 20kw more power, 5nm more torque, and the sky doesn't weigh THAT much more than the sil. Therefore it has the potential to beat the CA18DET silvia. The linear power delivery from the RB25DE also gives it an advantage.
remind me who wasn't very bright and needs to wake up?
Stock for stock I'd place a bet that they're fairly even.
ps. i thought about the torque band a turbo would give you as you suggested, and then decided that it was crap.