Whats the shipping weight of a standard boxed empeg? (with sled etc)

It's difficult to say - a lot depends on the packing.

As an example, I've been shipping a few empegs recently. All had identical outer boxes.

1) Original empeg inner box, bubblewrap and cardboard between the boxes - 6lb 8oz.
2) Original empeg inner box, foam peanuts between boxes - 5lb 10oz.
3) Non-original inner box (smaller), foam peanuts - 4lb 15oz.

That's a pound and a half difference based on packing alone. It's clear that the weight of cardboard dominates the packing weight, so the size and construction of the boxes has a large effect. These units were well packed in reasonably large sturdy boxes - I doubt anyone would be going larger/heavier. I'd suggest that the 2nd scenario is a good indicator. Note that none had sleds in so you'd have to add that weight.

Someone has had a bit of practice with this...

I would advise that, no matter what you do, make sure you use an outer box and have the empeg box inside it with plenty of packing (peanuts, bubble wrap, whatever). This protects the original empeg box from damage, tape and labels. These boxes are getting more rare, and it is really nice to keep empegs together with their original boxes with matching serial numbers.