I've been using VirtualBox for years and running BOINC in a virtual machine with Linux or MacOS guest operating system on projects that run faster on their Linux or Mac applications.* Although now I sometimes use VMWare Player instead of VirtualBox as I found it a bit more efficient. So didn't vote in poll as I already had VirtualBox installed separately.
It was only recently when I first ran a VirtualBox project, Sourcefinder Duchamp application, from within a Windows OS host machine's BOINC. It used the same VirtualBox version that I already had installed. So that means that unless you are running a project that uses VirtualBox from within BOINC on the host operating system then there is no extra overhead at all, as VirtualBox doesn't run. In other words standalone BOINC and the BOINC that bundles VirtualBox with it are the same BOINC program. That is, VirtualBox is a separately installed program that is started by BOINC only when VirtualBox projects are run.
I don't know how other projects using VirtualBox work, but Sourcefinder Duchamp starts up a separate virtual machine within VirtualBox for each task that is running. As to how efficient that is or not, well it's a bit academic as if you want to run Duchamp tasks or other projects' VirtualBox tasks then there is no alternative.
Although Duchamp VirtualBox application actually worked fine for me I still have some misgivings about projects' compulsory use of VirtualBox. Some comments by myself and others about this can be found in this thread: http://forum.boinc-australia.net/index.php?topic=1950.0
So if any of the projects using VirtualBox applications interest you, then perhaps you can suck it and see. No harm done if if doesn't work properly or is unsuited to your system. Just uninstall VirtualBox and your BOINC install will remain intact.
Haven't been back to Sourcefinder Duchamp since the admin embarrassed himself by trashing the whole database, so don't know how its development is going now. When I get some new memory next week for Skylake box I may give it another go. It's an Aussie project so I'd like to support it if/when it gets going properly. I'd prefer to get stuck into it a bit more if stats were being exported.
Anyone interested in running Sourcefinder Duchamp should read the forum there first to check for current issues. In the past the VirtualBox version bundled with BOINC was not working there and a more recent VirtualBox version needed to be installed.
*For example currently DENIS@Home is much faster on Mac, POGS is faster on Mac and possibly Linux and project applications at World Community Grid and elsewhere based on the molecular docking program VINA are almost always substantially faster run on Linux.