I have seen those...At least they are more than 5 spoke ! Personally I prefer the Oz wheels but then I think the OEM design has a simplistic and pleasing symmetry that suits the bike very well. There are 5 spoke carbon offerings available. But they look wrong, to me ! Although, I have seen them look good on Gorilla's bike as part of a considered, all carbon package that included a carbon OS kit...
The rear rotor is made from steel, ground and heat treated for longevity..
It is necessary because the Aprilia disc is too small diameter. I seem to remember I kept to OEM thickness too ( stamped on your disc) I was happy to discover that getting one made to my spec was cost effective and quite quickly done here in the UK. Same with the sprocket, although I have a hard anodised aluminium offering. (As soon as Supersprox get their heads out of their arses and open for business to Europe as normal, I will migrate to a custom Stealth sprocket, again, because I dont expect the aluminium to last that long...!) I did this because I wanted to keep standard final drive ratios and chain specs. This ensures the speedo reads correctly. I didnt want to go down to 525 chain...!
If you are a machinist, you'll have no problems....!