Photinia 'Red Robin' grafting for a standard?

Hi all,
I keep seeing Red Robin and Little Red Robin standard and half standard trees for sale at quite heft prices.
I've made my own Prunus Lusitanica standard without much fuss, by simply keeping the leader and removing most of the leaves.

I've been wondering what stem stock is use for Little Red Robin standards, as I really don't think one would make a successful standard by simply removing leaves/branches and keeping a leader - it doesn't get tall enough.
I read that it is grafted.  As for general grafting I read to use a similar species.

Does anyone know what stem and have experience with grafting Little Red Robin or the larger Red Robin into a standard?

Thanks