All of my cages have been Custon Cages and they do the job right. The good thing is they now do a "Core kit" so you can select the specification you want so I think the 6 point bolt in kit comes out at around 280+VAT, but you will need to weld it together yourself.
i assume this means your keeping said corrado and finishing it like you want to? if so i think it's a good move. looked dam nice at gti trackshow.
Protection and Performance were at Autosport this year, me and PhatVR6 spoke with them. There cages looked very nice, proper mandrel bent hoops unlike a lot of cheaper companies. I think their quality will be up there with Custom Cages although like PhatVR6 said their prices are 1,200 for a multipoint (through the bulkhead) weld in cage for his Corrado. Bolt in 6 point cages from Heigo or Wichers will be around 700-850 depending on what you need. OMP cages are very poor quality and although Safety Devices are strong the design is not pretty, I don't like the way they bolt cross pieces to the hoops.