I’m aware of how much potential power a fully built ka motor can make but i’m curious as to how strong the stock transmission is ie. how much power the tranny can hold before it starts to break parts internally.
ka-t.org home of 700+whp ka’s check it out. and use search buttons 
keep in mind that while most s13/s14 trannies are decent, there’s still plenty of them that have input shaft bearing problems… and that’s on stock cars w/o turbo kits… so it’s a gamble really, same way you are gambling w/ that turbo on a stock KA (w/o doing the bottom end).
like lucidsnow said, do a search on ka-t.org and you’ll find out soon enough
There’s also what you do with the car, if you have 275mm drag tires and you launch the car every weekend with a race clutch, it won’t take much power to snap something.
reliably you can expect 400hp from the fs5w71c assuming you don’t slam gears and don’t use a shocking clutch like act… try something like sbc/dxd or maybe eurodrive that engages smoothly
also the shift forks tend to break if you’re rough