Mark Elliot used a high Helium content for suit inflation on his record dive to avoid counter diffusion issues.
He strongly believes its a major issue in deep diving beyond 100m but I am not qualified to give detailed synopsis of his case I can only direct you to his web site. I have a beta copy of his new decompression soft where which uses stuff like 50/50 for deco. It’s hard going but interesting when you can get it to work.
http://www.inspired-training.com/
ATB
Mark Chase