I tested RL-26 in my 18" 6.5x47, It just didn't have the case capacity to get enough in there.
My results below of RL-26 vs RL-17 (Keep in mind this is an 18 inch tube, so your results may vary)
RL-26 with 140 amax (measured with Magnetospeed)
41.0 = 2463 fps
43.0 = 2561 fps (this was a compressed load, couldn't fit much more in there)
RL-17 with 140 amax
39.0 = 2622 fps
41.0 = 2780 fps
So based on my experience, I would run with RL-17 or 16 in the Lapua.
The RL-26 was awesome in my 6.5-284 though, real good velocity in that.