The 20" and 21" argument isn't everything to your idea. Going 21" you'd have to reduce the tire profile to stay within the rolling circumference and not get a misreading on your speedo. the 235 width is more to do with what fits on a certain width, and apparently there is a "standard" (the scandinavian version is called STRO, there is a European version as well that I can't remember the acronym for) as to what's the minimum you can fit to a certain width wheel, taking profile into consideration, how hard your local law enforces this may vary, but I learned this the hard way.
That being said, what's more pertinent to your question is the wheel width, and also the wheel offset. From the few TT wheels I've seen they have a rather weak offset which will lead to the wheel sitting fairly recessed in the arch. The A5 has fairly roomy arches, and IMO also a "low" offset from factory yet still has the typical OEM recessed look to the wheels. I would hazard a guess the TT wheel width and offset would be, at best, 9" and 35mm offset. To give you an idea what an A5 "needs" for a flush look you're looking at a 10" wheel and offset closer to 30mm.
All that being said, I don't think your car looks weak in your pictures, but spacers is an easy and "cheap" way to add aggressiveness to the fitment and stance. But it's a slippery slope, because as you push the wheels out, the gap between the tire and the arch will make your car look more like a giraffes neck and next thing you're looking at lowering the car for the ultimate stance look.
Here's mine moderately to aggressively lowered with 10"F and 10.5"R wide 20" wheels, offset is fairly aggressive to at below 30mm (can't remember them at the top of my head atm).
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hi thanks for the response, I think in the mean Time I will put spacers on as I paid close to a thousand euro for the TTRS' , so I think for now I will try spacers then maybe in the summer I may change the rims, I will post a picture once the spacers have been put on, again thanks for the detailed response.
Regards