question for the multitablers

[supersub]

When I was full tabling i played 8 tables with relative ease. Now that I am playing 6 max I jumped down to six tables.

My winrate is 1.35/100 over about 40K hands at 3/6 and 5/10 and even if I feel that I have been running somewhat bad I am starting to wonder if that really is the case.
could it be that I am losing a lot of value due to the fact that six tables is just to much for me to handle?

I calculated that a my winrate would have to be about 2.5/100 to equal my current earn if I cut two tables and just played four.

what are your thoughts about multitabling and winrates? where do you think the critical point for the average descent player is? has anybody done some research on this?