It's basic physics - a lighter rider would increase the power to weight ratio of the machine and therefore SHOULD result in a faster race. The nuance to this is that there is a balancing act between drift and traction that makes our sport as beautiful as it is.
I remember hearing Woffy say in an interview about 12 months ago stating that riders like Zagar would never be world champ because of how tall he is, but compare him to Vaculik who is arguably just as tall walk away with a Bronze medal in 2023.
I think it's worth investigating, not necessarily implemented, but it would be interesting to see two riders going head to head (for instance Ben Cook vs NKI) then see what the results would be if they added weight to Cook's bike to make the weight even between the two.
My opinion on it is that it probably does have an impact, but arguably not as much as some riders are making it out to be. Engines can be tuned, timing can be changed etc to suit riders and their build. Screeny was always one of the more heavier set riders towards the end of his career and he was still dominating at championship level up until his retirement.