Forged = mega money, that's why Porsche discs make people cry. I'm not sure if any are available for the MKV.
Grooved cast discs are perfectly fine, it only needs a few cuts to shift gases. Alcon, AP and some others have all moved to cresents. Look at discs on race cars and you'll see what works in the real world, not the "tuners".
The more grooves you have, the more wear on the pad and more noise given off them.
ATE's 2 piece discs aren't there for the MKV yet. There are some
Zimmerman versions though (again highly regarded). Note the use of an aluminium 'bell' and the disc on pins to allow expansion. I know they're drilled, but there's big gaps between holes, I'd be buying these if your budget stretches because of the 2 piece benefits and chance your arm on the cracking. ECP sell these and have a great warranty policy...so hopefully you'd be ok if they did. ATE if not.