Regarding compiz performance:
When compiz is enabled in any of the Vbox 3.0.x series, including 3.0.6 beta with 3.0.6 guest additions, there seems to be an 1 to 2 second keyboard lag. When using VBox 2.4 using exact kernel configuration, xserver setup, compiz version, default plugins, 2.2.4 guest additions, there is no issue.
This issue is the same as that presented in bug Ticket #4653 (Final frame of Compiz animation not updated to the screen) and also in "3D performance under Ubuntu 9.0.6" posts under linux guests
System performance is degraded when running compiz in VBox 3.0, VBox 2.2.4 does not present this problem.
Host :
Vbox 3.0.6 beta 1 release
Windows Vista x64 SP2
Intel Q6600 @2.4
NVIDIA 8800 GT
Guest:
Gentoo Linux x64 with KDE 4.3.1
Guest Additions installed