The Red Pros were slightly handicapped in relation to the Ultrastars by being 8TB rather than 10TB drives, yet more than held their own or even surpassing the higher density drives.
No real evidence of the Synology 'secret sauce' with their bespoke drive firmware but the tests run looked to be hobbled by the non-native 10GbE adaptor as well as the overall performance of the client laptop.
The MSRPs of the drives tested are all within the same ballpark but as noted, the Red Pros typical retail price is way below the others. Still, all a bit academic with the vendor locking to the Synology-branded drives.
[Not sure I should be commentating as an unreformed shucker.]