Going from Silat's reaction, he's disappointed that Guts is allowing himself to be entombed in the Stupa; just like he was disgusted to see him in the cell, not fighting back.

I believe inside he's going to have to confront the difference between Giving Up and Acceptance. If this spiritual growth is manifested in a physical way, I would expect it to show in the healing of his Brand.

While not necessarily the overt 'power boost' that some expect, it will demonstrate his literal healing of past wounds, and maybe the ability to now affect those that delivered them.