I have good news to report!
We expanded our test LUN to 1TB, and waited for it to sync to the other site.
I ran a test recovery plan (this time with 2 VMs) and it worked fine and finished in 15 minutes!
It seems like it doesn't really matter how big the LUN is, or how many VMs there are - it takes around the same time.
I will try a real failover next - and report back. All my versions are listed previously - but if this is all successfull, I will summarise with all the versions that I'm using.