Awright chaps,
The great drilled/grooved debate will rage on for ever.
I can have any discs drilled, when it comes to solid lumps of cast iron then as opposed to the Mk4 R32 disc which was semi floating and a patented design and lightweight, the disc is in reality nothing special.
I use either SBS or Bremtech aftermarket discs which have a good carbon content, they are cast in the UK which is the home of foundry ISO standards so good quality material is assured.
Discs very often are neglected, either electro nickel plating when new or powder coating the exposed surfaces should eliminate rust which can cause hot spots, which then causes problems with pads which in turn causes problems with discs and the circle of crap braking continues.
Basically -
Drilled look better, for track days better than plain - provide a gas path so fast road/track
Grooved ultimately better on track, can also act as a min thickness indicator when milled correctly - dont underestimate how noisy they are
There are interesting new styles such as J-hook and eliptical. AP racing are really revved up about these new designs apparently especially J hook.
My R32 discs are drilled as the R32 disc just isn't a track disc, its thickness would allow some favourable heat build up for a couple of laps better treat it as a fast street/occasional track day disc and drill it accordingly.
Its important that any CNC drilling program is well though out and the pillars internally are avoided, on a straight vane disc, invariably this leads to a curved pattern which is what I ended up with on mine which are in addition painted brembo grey around and up the bell and on the edge to prevent corrosion, they're also etch primed under the powder coat.
Its also imperative that the drilled holes are machine countersunk, ideally with a curved cutter this should eliminate cracking but a few cracks are acceptable.
Its just a different program to mill some grooves as opposed to drilling.
More so than ever I wouldn't be hung up over OEM discs on this particular disc or model save your cash for better pads, this is where most of the work is done. Mikes right you could have the best pads in the world if you dont bed them in properly then you're going backwards before you shut your door