hi ts,
if u are shooting distant (agar agar 100m or more) landscape, why not use your existing tammy 17-50 (very sharp lens) to do a series of sweeping photos and stitch together to form a pano?
there is a very good stitching freeware call "HUGIN", much better than the auto-merge function of some popular image editing software. although this method may not give you 100% of the kind of distortion found in ultra-wide angle lens ("HUGIN" can input distortion after stitching, but i dont have a comparison of it with UW lens), it does offer an alternative without spend $$$ (times are bad).