Just to plagiarise an excerpt of the response on the US Supra forum;
Main cap bracing has been done since the 60s with incredible rates of success. Cosworth used to do this in the early turbo charged F1 days, and as we know, F1 engines tend to live a hard life.
The brace kit works by adding rigidity and strength to the main cap, as you would expect from billet mains, but without the need to machine the block.
I am sure your local engine builder can tell you about at least one destroyed block that was sent to a machine shop in order to fit a set of magnificent billet mains only to be returned with imperfections and defects which ultimately rendered the block; a heap of scrap metal. Sadly, whilst our engine builders are up to par with their American counterparts, our machine shops are no where near as good.
This brace kit does not save you any money really, but it might just save your block from being ruined by an inexperienced machine shop.