I don't know about their Welsh Harlequins. Get a hold of a SOP and compare your ducks to it. If you think they are close to the SOP, take them to a couple of shows and see what happens.
The only ducks I know about for sure are the Blue Swedish and Metzer's is very inconsistent. I've seen Blue Swedish from there that are knock-your-socks-off gorgeous. I've also seen Blue Swedish from there that have misshapen blotchy bibs, green heads, neck rings, have the wrong shape heads, and are over-size.
So, I think it is possible to get show quality from Metzer's, but you are gambling about what you will get. It might be really good, it might be hatchery quality, and there might be both types in one breed in the same delivery.
They are very nice people at Metzers and they really care about their birds and their customers. I suggest that you call them and ask if they have any show quality Welsh Harlequin.
To me, the Exhibition WH are pretty easy to identify because they have unique plumage and that gorgeous dark bill. Show ducks should look very smooth and all the parts should flow together into a balance whole where everything looks like it was custom designed to fit together perfectly. The colors of the plumage should be very clear, distinct, intense, and glow with health.
I seem to remember that Metzers makes no claim that their birds are show quality. Maybe when they start selling Holderread Buff Americans, they will sell those as show quality. I know that they are keeping them separate and breeding up the flock without selling any, yet. I'm watching with interest.