First off: Before you do any mods, make sure that the engine is in good condition. Oil pressure, bearings, exc. Also get a boost gauge and measure your current boost pressure.
Secondly: Which type of engine do you have? Old or new? Reason is the turbo. Old type has the mitsubishi TD05-12B turbo, and the newer engine has garrett T25. Mitsu turbo is a bit bigger, and it can hold more boost, while the garrett has better spool. But either way, I wouldn't go above 1.4bar without other mods. I would strongly suggest that first you remove the pre-cat which is right after the turbine outlet, with a custom downpipe, and also remove the cat. This engine has a very restrictive exhaust, so do that first, and then you can raise boost.
But you can also do a turbo swap, with either a mitsu TD05 Big 16G/TD05 20G, or a garrett T3. But bear in mind, that some custom fabrication will be required to make the new turbo fit. Stock IC is not bad until ~300bhp, but having a FMIC unit will improve the intake temp. a lot.
I don't know if there is anyone out there, that can claim for certain... "For this engine, XXX bhp is the max". But generally all the parts (injectors, fuel pump, IC, turbo, pistons exc.) are good from around 280-320bhp. Of course, it can vary. Conrods on the other hand, as I've heard, are good for ~500bhp - but other people can confirm if this is true.