I have exactly the same rig (and a photo taken from almost the exact same position at Preah Khan!) My solution to the edge falloff is simply not to use the full width of the two frames and crop off the bad bits. I end up with a image 50x24mm which I can live with.
Question about stitching, if it's not OT; how do you use the stitching software to flat stitch? It never works for me, I'm using PT Assembler, and it needs things like roll, yaw and pitch which are not relevant for flat stitches.
It's a small world, isn't it . I've got lots of shots of that particular scene, some using the 20D with the TS-E 24 lens, some at 24mm with the Canon 24-105 IS and 5D, and some with the Sigma 15-30 at 15mm. There was a problem with the weather when I was there. The sun kept breaking through the haze and clouds for just a minute or so at a time.
Panavue was the first stitching program I tried, years ago. It seemed one of the best at the time and I've stuck with it. In the meantime, they've improved the program greatly. It now handles huge
files in 16 bit whilst also preserving the embedded profile. Having up to 8 pairs of flags at each overlap also helps with difficult images.
I've attached the 'Options' dialog box. Hope this helps.