I've driven 1.8TFSI FWD and AWD back to back. I owned the 1.8 FWD.
The FWD feels quicker and it's quite noticeable in between gear acceleration. The Quattro will be quicker off the line due to launch control and traction. but once moving the 100kg lighter FWD is more nimble and faster. The FWD also has a better top speed (according to the brochure
Note: the FWD is not only lighter, but the 7 speed DSG is bit more efficient (i.e. it doesn't lose as much power) vs the 6 Speed - Wet vs dry clutch
That´s completely wrong.
1.) What you believed to feel is misperception, because everything just goes softer/smoother and more elegant in the Quattro model. But no way slower. The 1.8 TFSI Quattro is the stronger car having more torque with 280 to 250Nm. You wrote that in measurements the FWD has ~280Nm but sorry mate, as a tuner owning a project 1.8 TFSI Quattro I can tell you that it also had 306Nm measured when stock. Also it has the *much* better DQ250 6 speed DSG which shifts smoother because it`s not the cheap "compact car" dry clutch but the more upper-class oil clutch and so your argument "wet vs dry clutch" is pointless as all better DSG´s excepting the cheap compact car DQ200 are having oil clutch. Look into german forums where you can find several threads about the jerky starting and hard shifting DQ200 which audi only used to save money for smaller engines like 1.0/1.2 or 1.4 TFSI (I agree that they should have given the DQ250 to all 1.8 TFSI models and not only the Quattro).
2.) 0-200 km/h is 30,2 sec with the Quattro and 33,5 sec with your FWD. Measured with to stock cars from germanys largest Audi Zentrum in Dortmund. So tell me, where is the FWD "more nimble" or "faster" now? The top speed is ~4 km/h less, thats right. But that only due to shorter gear ratio in the top gear.
3.) It´s not 100kg heavier, only 85kg to be exactly. Which is more then equalized by the better torque and better (quicker shifting) transmission (which does NOT loose any more horsepower, it just needs more VOLTAGE and therefore FUEL, thats a difference and does NOT affect wheel power in any way).
4.) You wrote "...4 wheel drive loses ~+-30% power at the wheel, FWD loses ~+-15% power at the wheels..." Now, thats the next wrong fact. It´s a myth, a relict remaining from the old TORSEN Quattro still used at larger Audi models. Torsen is always active 50:50 whereas HALDEX used in the A3 Quattro disconneting the rear axle when no wheelspin is detected. And at speeds >70km/h >45mp/h there is no more wheelspin so on every highway or Autobahn the Haldex Quattro model will not suffer from any AWD power loss. Because it is a FWD car in all those situations and speed levels.
If you watch Youtube videos like that one
you will realize how much more powerful the Quattro accelerates even from 100-200 km/h. It´s circa 18 sec (speedo) compared to usually 21-22 sec (speedo) for the AWD model with less torque. And torque means everything even at Autobahn speeds. The Quattro allows 30 more of it due to his MUCH better DSG (DQ250 tuneable up to 550Nm and DQ200 maximum to 330Nm). Oh, and as i am a tuner myself i can tell you that most of the A3 1.8 AWD I had here we measured with average numbers like 188, 183 or maybe sometimes 191hp but never with the 200hp you meantioned. Whereas the few Quattro models we saw measured at 203 and 194 and 191hp...Audi selects the better performin engines for their Quattro models to compensate the 85kg more weight, that´s an open secret and even a good contact in Ingolstadt (audi engineering) confirmed that last year.
Now, due to it´s much better potential, our A3 Quattro has 300hp and 420Nm Stage 2 which we could never have done with the -sorry- ****** AWD S-tronic model. ****** because having poor traction at traffic starts and even more poor DQ200 dry clutch that you will not find in ANY bigger model. It´s a big fortune for every 1.8 Quattro owner that Audi built the grown-up oil clutch of the S3 into that model.
Cheers