I would not run that turbo without supporting mods and properly mapped by someone that knows what they are doing, otherwise any 'safe' boost level is simply guess work.
You'd be far better off getting a 2nd hand standard turbo, and running that until you are ready to do it all properly.
For what it's worth the stock pump is on it's limit around 300bhp assuming it's getting good voltage to it, slightly more for the stock injectors unless you raise the fuel pressure a bit, then they will max out at 340 - 350 bhp or so.