If "impulse" buys are truly their concern, they need to hire some more reputable firms dealing with sales consulting. The prices on Games Workshop products lodge them firmly in the luxury range. Only a select few people "impulse buy" luxury items. Most of us have to think long and hard about dropping the kind of coin their product requires. In short, their product is not good for impulse sales in the first place.
If you doubt me, go to your local store (any level from Walmart to Boutique) and look at the items placed by the register. These are the ones intended as "impulse" purchases. You will find that these are the cheaper items that particular store sells. By contrast, most companies that sell luxury items start an advertising campaign which hits the public long before the item itself is available. This is because they want people drooling over it and thinking about it and working it through in their minds.
I find the whole notion of "impulse" buys laughable, but sure let's go with that. Explain how seeing a picture of an elf on a bird a week early will allow this person to change his/her mind?