Turbochargers need to be perfectly balanced or they will self destruct. If you are considering an upgrade, there is machining to be done to the housings too. Get it done properly by a company which specialises in such things.
With that in mind they will also be able to advise you on which turbo, upgrade, internals, mods etc will be best suited to achieve your goal.
Don't forget that by fitting a bigger or upgraded turbo you will require the additional fuelling to suit, more air requires more fuel which in turn can require timing adjustment depending on how far you go, it is better to control that than not.
An upgrade will also need control to regulate boost, and the additional fuelling. it is highly unlikely that your standard ECU will be able to regulate the larger injectors that will need to be fitted to assist with the new fuel demands.
On top of that, there is fuel delivery, your stock fuel pump may not be able to deliver the amount of fuel to feed the injectors. Then there is fuel pressure regulating, your stock FPR may not be able to hold a steady pressure.
I don't want to put you off upgrading, far from it but i do want you to understand that it is not a straight forward thing and the costs snowball.
Do your research and base your upgrade path on your budget.
As for how much BHP it gives, this is down to so many things, the size and flow of the turbo, which injectors, which route of cooling the air etc etc etc.
There is just so much to write here that it would take ages.
First and most important though is starting with a good, well serviced engine that has good compression and a solid bottom end. If not you will only bring about an early demise to it and be left with a bag of scrap metal.
We often have people phoning the workshop wanting 500 bhp and a budget of £500, it's never going to happen, be sensible with your choice and don't overlook the insurance hike too.
Good luck.