When you start to think about upgrading to a Bigger turbo there comes the Bit n Bobs you need to upgrade such as Injectors etc. Unless you are looking for a massive increase such as 280bhp+ then yeah go for it but it gets so expensive. I was looking into it last year but due to the economy i chose to save my pennies.
Your looking at 240bhp from you 190 edition, keeping the same type turbo with some subtle engine mods like TT injectors/FMIC a Custom remap then you can come close to the figure you want. Its not just BHP you want to be looking at, its the Imortant Torque...
Check out BUBSTER's spec, he's running a good figure with both BHP & Torque with some good Mods.
The Gt28 is capable of some massive gains and paying what you would pay for a brand spanker is going to waste IMO 'unless' like I said - You are going for a Massive increase in Power. If it was me, having the GT would be pointless unless you are going to take it a level higher - again, becomes quite expensive. If you dont mind spending a couple of Grand then go for it.
Its not just about geting a bigger turbo tand thats it - its about having a Clutch that can take the power & then it comes to the Brakes, after all you want to have decent brakes to stop you from all that power hey!!!
Id stick with the same type turbo, get a uprated Turbo Pipe, add a FMIC (as they can be done pretty cheap) and get a decent Diverter valve (recirculating) and get a Good map on there (somewhere thats into Custom mapping) and away you go.
...oh and get a decent Exhaust, milltek maybe.
Which ever way you look at it, to acheive your figure, you will still have to spend a few quid for decent materials!