My Speaking score jumped from 6 5 to 7.5 with IDP. However, I was confident that I did well and the speaking examiner was too rude.Well, IDP is far quicker than BC in terms of reval results. You shall get your results within 8 weeks.
Regarding success rate its difficult to say , but it would be around 6-7%. Also, there are many who did not get desired 0.5. You can check through different threads for more information.
I feel they don't care about giving honest marks to candidates. They give randomly based on their mood.