Guys you can’t just plug in a raytrace card and then tell a photon based offline renderer like lightmass to just use GPU power. You need to rewrite the renderer code almost base up to enable brute forcing or lightmass like support plus make it GPU compatible, at which point might as well replace the entire aging renderer with a new one.
Lightmass is the same story as Mental ray it uses similar underlying tech like final gather this accounts to all the inaccuracies in light quality and bounce, light bleeds etc… In the end they had to kill mental ray because its cheaper to write a whole new renderer from scratch these days than beat a dinosaur.
Also as many suggested here RT is just hype these days, it will be many years until it can be properly used or integrated if its not replaced by then by something more hybrid but with equal or better results.