The 1.4T is not the same sort of engine as the 1.8NA, or any other NA engine. It doesn't kick you in the pants when you step on the gas, there's a small lag while the exhaust gasses rush out, spin the turbine, forcing more air in, which allows more gas to burn in the tiny cylinders, causing more exhaust, more air density, more gas... it builds. It builds relatively rapidly, but it does build. It also doesn't really dig in until higher in the rev band than you may be used to. This means if you shift early (or short), you might never really see the little engine that could dig in and kick you in the butt. When I test drove the 1.4, not knowing anything really about this, I could not tell it from the 1.8, to be honest. It was, if anything, a little wimpier.
This is, however, exactly the point of the 1.4, and the reason I said earlier that it is anemic. If the turbo never kicked in (and apologies to those global citizens who can actually buy a 1.4NA Sonic) then the car would barely have enough power to get out of its own way, it would slowly come up to speed, cruise around with a relatively low but I suppose viable top speed (say 70mph or so?) and it would whine all the time. Drive the car like this, which you can by shifting early and keeping your foot relatively light, so you never get enough exhaust pressure to bring the turbo up, and you get pretty good mpg, we've seen people get as high as what, 65mpg? Far outside the EPA's measurement, which isn't done by driving carefully.
The Turbo, though, means that if you want to get up to speed quickly, so you can merge or pass or whatever, that you can stay in each gear longer and dig in, get your boost up and have the equivalent of a much larger engine... 14.9psi is one atmosphere at sea level, more or less, and even stock tuned the 1.4T can get up to that much overpressure, which means, essentially, that you've doubled the amount of air crammed into the cylinders; you have, at full boost, the equivalent of a 2.8 liter displacement engine.
With a tune taking the factory limits off, you can run up to 22psi, get there quicker and more smoothly. That's similar to a 3.6 liter engine. But most of the time, you're still only using that little 1.4 engine, or perhaps if your cruising speed is over 65 or so, a 1.6 (with a little boost going to keep power up high enough to cruise at those higher speeds)
The AC does matter, again precisely because the engine is so anemic. It's good for maybe 5 hp. Putting hundreds of pounds of cargo/passengers in counts too. I can tell you I have much less "sportscar" feeling when I have an extra thousand pounds of payload (that's my son and three of his friends, all well over 6 foot and 200 lbs).
All that said, though, the 1.8 is a good solid engine that does the job more than adequately. I think you'd find even untuned the 1.4T does so still better, once you got used to the difference in driving style... but it probably doesn't do it $2400 better. I paid only $1700 extra for mine (as I would have been happy with the LS), but even so, if I wasn't so enthusiastic about the idea of a deliberately wimpy turbo, I probably would have stuck with the 1.8, and been happy with it.
