I'm actually working on modifying Toga II to be a "sparring partner".
To do that I'm making some tests now but I think I'm near a correct elo decreasing formula without decrease the calculated nodes or slow down the engine.
As an example, The 2500 plays against 2200,2300,2400,2600,2700,2800 (+/- 300 elo).
Here are partial results calculated with bayeselo :

What do you think about this ?