If you want to see the ROIs achieved by some mass multitablers, look up the players on the Sharkscope leader board (or on one of the other similar tracking sites).
Some of these players have played a large enough sample that their ROIs are reliable within +-2%, at least for past game conditions. It takes tens of thousands of tournaments to get that type of accuracy from the results alone.
Of course, it probably took these players talent, effort, and time to learn to play as they do, so don't expect to reproduce those results instantly. They do show that some FAQs for achievable win rates are way off, perhaps based on softer games in the past, or when players thought 2 tabling was normal and 4 dangerous.