1.8 tsi owner here. Any my mum's got a GTD so I know what the diesel 184 feels like, as they're essentially the same car.
The way the throttle is mapped on the 1.8 tsi is relatively long. You have to give it a lot of gas, but when you do it will deliver. My mum’s GTD kinda gives it all up front at the start of the pedal travel. This gives the impression of lots of grunt (which there is), but there’s not a lot past that point. Maybe with Carista / OBD11 you can change the throttle pedal settings? Not sure about that one, sorry.
So, why does the 1.8 tsi feel comparatively slow to you?
The 184 tdi does have significantly more torque, 380 Nm to the 1.8 tsi's 250 Nm. However, you need to look at where this torque is delivered.
The diesel car dishes out it's 380 NM from 1750-3000 RPM. This means you have
93 HP at 1750 RPM and 160 HP at 3000 RPM.
The Petrol car has 250 NM from 1250-5000 RPM. This means you have only
61 HP at 1750 RPM,
105 HP at 3000 RPM, but
175 HP at 5000 RPM, where the petrol power takes over and gives you its full power until 6200 rpm.
That's a decent rev band to work with on the petrol car. With the diesel, you only have max power from 3500-4000 RPM.
In summary, the diesel feels faster down low in the revs because it has about 50% power from 1750-3000 RPM. It
is faster down low. However, the petrol comes on song later in the revs, which is when the diesel will have run out of puff and need another gear. The longer you can stay in a
lower gear, the more wheel horsepower you'll have due to being in the
lower gear.
My advice, use more of the rev range. I feel the car has pretty nippy performance, but you have to wind it out to get it. I like that, but I guess it's not for everyone.
I hope this at least helps you understand why the car feels the way it does. If you already knew this, sorry!