I had the same issue with rear disks. I did made the adaptor. 25mm inner to 27 outter diameter. What is the corsa gp you mention? Also Neil, what disks you have used?
The get easily to 230 HP, stock engine/turbo. Their traction control is not fully disabled, but when "off" and you do a drag launch, it acts like launch control and takes off really well.
Just for the lol, the oz wheels that evo 5 was rolling from factory were indeed enkei and not oz made. And look a lot like the wheels in question here...