there was an option on the vs senator of a twin piston caliper which was the same as the vn sv5000, vn groupa and the vs gtsr which had a 328-330mm rotor or the standard single piston caliper which had a 280mm rotor.
Standard VS (as used in Clubsport and Senator 5l) were 289mm ventilated at the front and 279mm solid at the rear. 5.7l models (GTS and Senator) had 328mm ventilated front and 280mm ventilated at the rear. Replacing your existing rotors with slotted ones won't make much difference to your braking efficiency. If you want better braking your choices are to:
1. Find a second hand set from a VS 5.7l Senator or GTS (very rare and very old)
2. Purchase new equivalents for Option 1 from PBR
3. Purchase VT front rotors/calipers/pads and retain standard rears
4. Purchase AP Racing (or equivalent) four piston calipers/pads and 330mm rotors and retain standard rears.
I took option 4 about 14 years ago now and have never regretted my decision. The standard Commodore VS brakes (unfortunately) are marginal.