A turbo upgrade is not what you are looking for for your first mod, start smaller and work up. However the turbo you chose isn't going to be making to much power for the internals if you have a good dyno tune and no detonation. I know of local guy making around 425hp on the stock internals. The problem he ran into was in the transmission. He shredded 2nd gear and ended up getting a STi 6speed swap.
The first upgrade for power I would do is a downpipe. You will feel the increased power and your the car will start to come alive.
The second mod I would do is an uppipe. There will be minimal power gains from it, but by removing the cat in the stock uppipe, you will ensure that you turbo will not be damaged later on from pieces of the cat that break off and go through the turbo. I also strongly suggest the uppipe at the same time as the downpipe, because during the install of the uppipe, you have to remove the downpipe.
The third mod I suggest is some kind of engine management. This will adjust the timing, fuel and boost allowing another significant increase in power. You can also have a dyno tune which extracts the most HP rather then running an off the shelf tune.
The fourth mod I suggest is a turbo, but by getting a larger turbo, you need more fuel and you need to cool all the extra air that the turbo is pushing. You need to get a larger fuel pump that flows more fuel and you need larger injectors so you are able to push more fuel into the cylinders. You also SHOULD upgrade your intercooler to a larger topmount or a front mount. You also strongly suggest getting a dyno tune to make the best of these mods.