According to here:
[The twenty percent rule] applies only to someone whose income does not provide generously for his family. Someone who has a job or business that provides adequately for his family is permitted to give everything above his needs to tzedakah even if it is more than twenty percent of his income or his holdings.
And from here:
Apparently, when Rav Yosef Sholom Elyashiv was asked he replied that he permits it. When he is summoned to the Heavenly Court, he added, he will present his Halachic reasoning for this ruling. If it is accepted, well and good, and if not – he is willing to accept upon himself the consequences!
I strongly associate with the latter view.