I could be wrong but doesn't sodium percabonate break down after 4 hours? I'm curious to know if that applies to sodium metasilicate and sodium metabisulfite as I use them also.
Sodium Percarbonate will disassociate (slightly more accurate than "break down ") into Peroxide H2O2 (that's the steriliser) and Sodium Carbonate (its the cleaner, AKA Washing Soda). The rate it happens, how effective it is and the amount of time the Peroxide is available is largely temperature dependant, as with most chemical reactions the hotter the faster.
Sodium Metasilicate helps Percarbonate work better, its helps remove soil and acts as a wetting agent. One other effect is it acts to protect metals, particularly Copper, Brass and Aluminum, from the effects of strong alkaline cleaners, worth thinking about if you are using an Al pot or have brass fittings in your system. Metasilicate is a common admixture in "Brewery Cleaners" PPW,
BBW...
The cleaning action is still going on while there is Sodium Carbonate available (yonks) the peroxide is pretty unstable so yes you would be brave to assume it was an effective steriliser for long (personally I wouldn't think so after half an hour).
If I were using Perk as a soak clean (which I do) I would use a finishing steriliser. I'm a lot more careful what I regard as a no rinse sanatiser so properly diluted pure Phosphoric, Peroxide, Ethanol sure, stuff with detergent and halogens (F, Cl, Br, I) no way so a no to StarSan and Iodophor.
Mark