Tips for running OVZ inside KVM effectively

So its come up a few times about running OVZ inside KVM.... @chief mentioned it previously a few times, most recently as an example way to 'bundle' your OS to make migrations between providers easier (in the context of those paying month to month).

The google summary seems to be: its doable and performance doesn't have to suck - but was hoping peeps that have experience doing this can share any tips.

In terms of efficient IO, is VT-D passthru possible thru all 3 layers?)

Assume a single IP for now (so port forwarding or similar network setup).

Thanks!