I've learned recently that High Quality and Ultra shadows are only saved in your local cache. They're not saved in the GBX file with the track. This means, they aren't sent along with the track to other people.
So, it would seem to be pointless to calculate High Quality shadows for other people.
Still, for some, even normal shadows take way too long. So this thread will remain open for those who need normal/default shadows calculated.
You can offer your services here for that. Thanks to everyone who has already offered.