Found a great Whitepaper on Virtual Hard Disk (VHD) Performance.
The following paper is focused on giving in-depth performance data on Virtual Hard Disks. Comparisons are generally made between native, Window Server 2008 and Windows Server 2008 R2. When comparing Windows Server 2008 and Window Server 2008 R2, no virtual machines are used unless explicitly stated. The goal of this paper is to look at virtual hard disk performance and not Hyper-V performance.
Download the whitepaper on Virtual Hard Disk Performance here.