Microsoft has updated the Performance Tuning Guidelines document to include W2008 R2. It covers all aspects of the server operating system but I’m going go focus on Hyper-V here.
The guidance for memory sizing for the host has not changed. The first 1GB in a VM has a potential host overhead of 32MB. Each additional 1GB has a potential host overhead of 8MB. That means a 1GB VM potentially consumes 1056MB on the host, not 1024MB. A 2GB VM potentially costs 2088MB on the host, not 2048MB. And a 4GB VM potentially costs 4152MB, not 4096MB.
The memory savings for a Server Core installation are listed as 80MB. That’s seriously not worth it in my opinion given the difficulty in managing it (3rd party software and hardware management) and troubleshooting it when things go wrong. “Using Server Core in the root partition leaves additional memory for the VMs to use (approximately 80 MB for commit charge on 64-bit Windows)”.
RAM is first allocated to VM’s. “The physical server requires sufficient memory for the root and child partitions. Hyper-V first allocates the memory for child partitions, which should be sized based on the needs of the expected load for each VM. Having additional memory available allows the root to efficiently perform I/Os on behalf of the VMs and operations such as a VM snapshot”.
There is lots more on storage, I/O and network tuning in the virtualization section of the document. Give it a read.