There's a mature 'Aureum' growing at Portland Nursery on Stark here in Portland that's a little above 8' but not too much...
Here's a picture of it (It's the tree right under the 'ENTRANCE' sign.

Here is a link that might be useful:

I would not consider an 8' 'Aureum' particularly mature :-) This tree has the ability to grow much larger than that.....as much as 20 feet in time. There is one in Belgium that is 150 years old and 26' tall! Around 15' would be a much better indicator of an average mature size. Yes, it does grow quite slowly but it can develop a decent size and faster than you might expect - I have one in a container that I've been growing for about 10 years and it is approaching 8' even under those restricted conditions.

Maybe the vigorousness of your container 'Aureum' is in part due to your very moderate zone 8 PNW climate?

I'm sure that plays a role but A. shirasawanum is a very cold hardy species and even MOBOT (the Missouri Botanical Garden data base) lists mature size at 16-20' so ultimate size is not necessarily a factor of climate. Just want you to understand this tree will get larger than 8' tall over time. Just how much time, I can't say :-)

FWIW speaking with a JM 'expert' (who is also in zone 5) is quite confident that it would take in excess of 15 years for an 'Aureum' to attain a height of 8' (zone 5).

But in any event it is for sure s l o w growing ;).