What a weekend! Have converted my 2 Hackintoshes to Opencore from Clover. They were both set up as iMac14,2 - so in the process of conversion I've generated new serial numbers and have changed them to iMac16,1 and iMac17,1 respectively so that I can upgrade them further to Big Sur when it comes. Opencore didn't agree with my Windows 7 install on a separate SSD (it reliably gave BSD) so I've put rEFInd in front of Opencore to fix it. Finally, after all was working upgraded to 10.15.7 which on Opencore didn't require any intervention. So far thing look to work fine, I've tested FaceTime with the newly generated serials - all fine. Boot time one one machine visibly faster (although it was very fast already with Clover), on the second slower machine (which acts as TV PC and router) comparable. No other issues encountered.