The OS Giken twin plates are one of the best, they are not aggressive at all and the pedal is only a little bit harder than stock.
The HKS ones are very aggresive and a bit of pain to drive with, but if its not a road car then it wont matter too much.
I run the Blitz twin kit which is nice too drive with, a little more agressive than the OS but no where near as bad as the HKS and the pedal is as light as stock.
A twin plate will set you back £900+ but will come with everything you need. They will all handle 600hp no problems.
Hope this helps.
Lee