What kind of gains can i get?
The lower power 1.4 TSI (122/125PS models) use the same size turbo as the 1.2 TSI, only the 140/150PS variants use a larger turbo.
Given that with no other mods, when mapped the difference between the two turbo sizes of the 1.4 TSI is roughly 30PS, we can expect the 1.2TSI should produce around 160PS (130PS being a common Stage-1 level map) with the larger turbo.
Obviously, with other mods like a sport-cat or intakes, this may well increase further - sometimes by more than 20PS, possibly achieving more than 180PS. Torque probably won't change much if at all though, it'll just hold that torque to higher rpms.
So, to summarise, with no other mods, probably around the 160PS mark, with torque somewhere around the 240NM mark.
Can I fit it without mapping it straight away?
Yes, but the larger turbo has different boost characteristics, it will run fine, but it barely provides any torque until about 2000 rpm, where the stock turbo provides strong torque from just 1400 rpm.
You will also notice that it feels like the engine is breathing much better beyond 4500 rpm, it just keeps on going where before it would feel like it ran out of puff. The turbo is barely doing anything yet it pulls so well.
This will all change when mapped - which i haven't yet done - and i'll be certain to update you all when i have had it done!
What else do I need to fit it?
You should buy a Gasket Set for it, this comes with new seals for the oil lines, charge pipe, coolant lines and new exhaust gaskets.
You can get one from here: https://www.autodoc.co.uk/elring/9970020 they take sometimes up to 2 weeks to arrive, but it's got everything you need in there.
Can I fit it myself?
Yes, for tools, you'll need your standard metric socket set, 8mm through till 15mm is enough, you'll also want a T30 bit, and a 6mm Hex bit (a T40 bit works if you don't have a 6mm Hex bit).
There's one more bit which is called a Triple Square Spline bit, it's like a 12point Torx, and i've no idea what size it is, i'll let you figure that one out for yourself! It's metric though.
There's 3 bolts which use that bit, visible from under the bonnet there's 3 of them, they secure the heatshield (which also serves as a brace), onto the turbo and the engine.
You'll want some vice grips (mole grips, moll grips, lockjaw grips or whatever you like to call them), to release the hoses off the intake pipes.
Finally, you will also want to lift the front end of the car on some axle stands, never work under a car with just a jack!
If you're not confident, just take it to a garage.
Tips for installation / Simple instructions.
First thing you should do is unplug the wastegate connection, that's just behind the back of the intake pipe as it goes into the turbo.
Take off the entire airbox (disconnect the flexi from the turbo intake, and the cold air feed - you'll need those vice-grips for this), it just pops on/off 3 nubbin type things, give it a tug and it comes off easily.
Take off the charge pipe, it's just got two clips on either end, chances are the rubber seal has probably stuck itself, so it might be a bit tough, but once you get it moving it'll come off fine.
Cover the throttle body! You don't want to get anything in there! I stuck a rubber glove over it.
Next, open the coolant reserviour to release any pressure, then follow the pair of metal lines round from the turbo till the rubber lines, move the metal clips an inch or two away from the end, and the rubber hoses will pull off.
Now go ahead and remove all the bolts for the coolant and oil lines (when you're underneath the car, there's a plastic/aluminium heatshield which can be removed by taking out the two bolts, you'll gain a lot more access by removing that).
Pull all the oil and coolant lines off (there's one in particular that's a bit tight - that's what she s....) from the turbo, so no you're just left with the turbo.
Unbolt and remove the heatshield/brace, that's those wierd 12-point torx bits and a couple random 10mm bolts if i remember correctly, and now the only thing left is to remove the turbo from the exhaust.
Now i'd suggest to loosen the exhaust clamp, but don't fully undo it, so when you take off the manifold nuts the turbo can't just drop off - as it's easier to get at the nuts from underneath the car, you don't really want a turbo dropping on your face!
Once you've got the manifold nuts off, you can fully undo the exhaust clamp, it might take a bit of tapping with something metalic to free it up, then you can pull out the turbo (from the top).
If you're replacing the manifold studs, now's the time, they're easily accessible from the top now. Install the shorter threaded side of the new studs into the manifold, the longer side *might* fit, but it also might bottom out (the threads about 25mm and the hole is about 25mm too), in which case you risk cracking the head!
Now just do everything in reverse, but with the new turbo, don't forget the exhaust gaskets! (there's one that goes between the manifold and the turbo, and one metal ring one that goes between the turbo and exhaust).
Putting everything back in was a doddle compared to getting everything back out. It took me 8 hours in total, and I ended up quite sunburnt! Don't go mad with tightening the oil/coolant line bolts, they weren't in particularly tight from factory anyway, if you're really worried put some loctite, just don't over torque them!
Manifold nuts and the exhaust clamp do need to be tight though, exhaust leaks can damage other stuff in the engine bay, make the engine not poorly (incorrect readings from the O2 sensors), and will fail the MOT if you left it that long!
Adaptation is advised but not necessarily required for the wastegate (I didn't do it on mine and it's been fine).
Before fitting, ensure the wastegate is able to open and close (it is a fairly stiff mechanism but it is quite possible to move it by hand),
If it's completely stuck then spraying some GT-85 both inside the exhaust housing where the wastegate protudes through, and in the actuator mechanism itself will work wonders and should reduce the chance of limp mode occuring.