Even while slogging through my painful and ongoing update of perl and friends, I managed to install Virtualbox-5.0.0 and its guest-additions. (Apparently these two packages don't depend on perl at build-time.)
I fired up my vbox gentoo guest machine and, of course, I tried to install the 5.0.0 guest additions kernel modules. I got an error message about usb support in the guest kernel, so I disabled guest usb support (temporarily) with the vbox gui, tried again and the guest kernel modules installed with amazing speed. I'd like to know if anyone else is seeing spectacular speed performance with vbox-5.0.0.