No answers from me, but some things for you to consider while deciding. As you pointed out, the 15mm can get reused on future DSLR bodies for as long as the K-mount lives; the GW-3 adapter might become unusable whenever you replace the GR with a new compact. On the other hand, the GR is smaller than a DSLR, possibly an important factor depending on how/when/where you plan to take those landscape photos. If you're going places where a DSLR is too large to travel with then the GW-3 looks great.
I generally don't like to buy many accessories for fixed lens cameras. Therefore, I would lean towards adding the DA15 to your K-3 provided that the K-3 will be able to travel with you. If I wanted to travel lightly I'd get the GW-3 or maybe just live with GR's native lens and stitch when I need to go wider.
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