Not sure re genuine HSV rotors but DBA X-drilled and slotted rotors are not heat treated after machining. I would think that distortion would be a problem if heated after machining, necessitating a final surface grinding step. (machining the disc surfaces of cross drilled or slotted discs is not possible).
The cracks around the holes in cross drilled discs are due to differential expansion between the hot disc rubbing surface and the cooler centre of the disc. The surface expands but is held in compression by the lesser expansion of the disc centre. The difference in size is such that the disc surface yields (in compression). Cracking occurs when the disc cools and the compressed disc surface becomes under tension from the (unyielded) disc centre. This occurs in undrilled discs but the holes weaken the disc and are stress concentration points. It is made worse by hard braking on cold discs without warm up application. Whether the cracks are a problem depends on how big they are.
Both holes and slots provide a means by which vapourised binder from the pads can escape. Holes also increase the surface area by which heat can be rejected, increase the area that air can flow through to the disc centre and generate turbulence in the cooling airflow which also helps heat rejection. The trade off is that the disc mass is reduced somewhat which reduces the heat absorption capacity for a single stop.
If drilling holes, they should be chamfered to reduce the stress concentration effect of the sharp edge of the hole. On solid discs, the trick is to peen the hole edges by hitting a ball bearing into them with a hammer, which also precompresses the metal around the holes. Not sure how a vented disc might be supported to avoid distortion using the same technique.
If labour costs nothing and access to the requisite tools is available, I reckon slotting your own discs is worth doing. Maybe look at a DBA disc for a pattern that works in terms of balance. Put it this way, if you are prepared to buy new discs anyway, nothing has been lost by playing with what you have already got.
