You're in Greece George, which seems like a foreign country to us. Shipping costs make those options not worth considering, for us, over here.
The Traction Concepts product and others like it are generally seen as cheap, low quality options, to get both wheels to spin. Not "real" performance products. More of a gimmicky work around, one step below a welded diff. I don't think I've seen anyone on any of the Z car forums I've been around actually buy one and use it. It's surprising that the product is still available.
One interesting thing about the Subie diffs are that there are two types, Torsen and clutch. So you have to make that choice, if it matters to you. Another thought is that it's actually fairly simple to change gear ratios, via a new ring and pinion gear set. The cost comes in having a good shop set the clearances back to spec. correctly, after installation.
One thing that's fairly common to do is to assume that the backlash and the bearing preload will stay in spec if only the carrier is changed. I know that certain skilled people on this forum have done that with R200's and been fine. It's common across OEM diffs, the housings are the wide-spec. parts but the carriers are typically precise and accurate. They can be swapped, across gears and housings. But then you can have those night sweats that gnosez gets.
I just went through all of this with a Ford diff, swapping an LSD diff and carrier in to an open diff housing. There's a bunch of different ways to get it done, but that means there's a bunch of different ways to make a mistake too. There's a ton of videos and instructions out there. It's a fun DIY, and even if you choose to have a shop do it at least you'll know what they're talking about.
And here's another option, the Cusco, which seems to get good marks. https://www.rallysportdirect.com/part/limited-slip-differentials/cus-lsd-182-l15-cusco-rs-rear-lsd-1-5-way-r180
Obviously, the work will make you chew your nails... eeeew...