The TD04 is
a single scroll unit. It is a rather small blower even for the 2L motor but there are different size compressors (which is what actually provides the boost pressure
) available as an upgrade but it's better to buy the entire unit unless you have skills. The turbine is the part at the back attached to the main shaft and is in line with the exhaust flow, thereby spinning the shaft and the compressor wheel. The factory unit has a small/medium 13T compressor but they are available with a 15T and 19T upgrade for the Subaru, each one being progressively bigger and having more puff. If you upgrade the turbo a remap will
be needed. Lots of info about that on this site.
Read up all you can on turbos before upgrading because they mostly behave differently and all too often the units that make big power and sound exciting do it all in mad rush above 3k. Right now, your factory unit provides reasonably seemless/unnoticeable boost, that is to say it's just there from low revs. The larger the turbine (usually, performance blowers have larger turbines that drive larger compressors, but not always)
the further up the rev range boost occurs as more exhaust energy is required to spin it up. The TD04-19T is an interesting one which I know works well with the 2L motor but there are others like the the VF28 and VF35 but these are quite expensive compared with the TD04. if you want to keep the low down but get more mid and upper power the 19T is a good way to go, some of the WRX guys who wanted more power but also needed to keep the car driveable everyday use them and it's where I got the initial info for mine.
This is as basic as I can make it, I had little knowledge when I started so have a look through my posts going back at least 5 years because I did the early experiments fairly blindfolded but was successful, it's all documented.
As to the boxer rumble, yes that 'silencer' for the back end should
release it alright but be prepared for some extra in cabin noise. There are a good many tail pipes available and have little effect on performance but some are nicely noisy where others will upset your neighbours and attract the wrong kind of attention, again, read up or talk to locals you come across that have modded Subarus which sound good to you. The "Boxer rumble" is as a result of the unequal length exhaust header pipes the turbo engine uses because they all have to gather on one side to meet the turbo. The 2 from one side of the engine are much longer than the 2 from the side where the turbo is located hence the unequal pulses and the rumble. The non turbo cars have 2 equal pairs that pass either side of the engine and join roughly under the footwell so no amount of straight through noisy exhausts will give the sound.
Good luck and welcome to the wallet squeezing world of car modding