There are a lot of factors that govern the monitoring of multi site Netapp gears from a single Unified Manager instance. There will be ~100 health ZAPIs fired every 15 mins ( during each polling cycle). And you have already mentioned the latency is around 300ms between the sites. Looking at all these factors it is advisable to have a seperate instance of Unified Manager per site.
I would also encourage you to read through the Best Practices TR here. Hope this helps you in planning your UM 7.2 instance deployment.