I print 8x10, 8x20, and 12x20 images with a 26-1k with no discernible fall off at all. 1.5 stops would lead me to think that the either the bulb was very old or the diffusion dots on the glass were gone, or both. I have heard of some fall off with people printing 20x24 with a 26-1k but no where near a 1.5 stop fall off. Mine seems to work fine within a 20" square. Although Dick Arentz probably has the latest and best exposure units available to him, if you look in his book there are pictures of him using a 26-1k.
The 26-1K I owned and tested had plain tempered glass beneath the bulb, nothing at all to even the light. The ones that have the pattern on the glass to even the light probably give better results.
However, if you are interested in testing this just expose four step wedges at the far corners of the frame and compare the results with one in the middle.