What about an out of tree driver? You develop your own version in your own repo without upstreaming your works. The FreeBSD guys used to do this and only upstreamed their works when they already have a working Guest Addition. Shame there is not yet an official version for FreeBSD as for Linux, but the unofficial Guest Addition already worked well.
Change Pointing device in VM settings to USB Tablet. This should fix lagging mouse. There was VirtualBox Guest Additions before but one of the update broke it so I removed it from the repository. Will try to find some time and at least try to revert it to a (previous) working version.
If someone is interested here is the recipe and patches:
Nope. It improve the situation, but the mouse still appears to be laggy. I already changed it to USB Tablet before I post my previous posts.
It already is an out of tree…
Problem is the patch got changed from my own version and now I’ll have to sort out the mess 
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