difference in performance on 16-19" tires
 

how will changing the size of the rims affect the performance?
gas consumption markedly higher?

When you pick different rims, you adjust your tires accordingly.
 

YMMV, but here's a general size recommendation. Obviously if you choose a more narrow rim, you will need a narrow tire:

225/50 16
235/35 17
245/35 18
245/30 19

You basically want to keep the wheel and tire in relatively the same in diameter.<ul><li><a href="http://ctny.audiworld.com/mark/a4/calculator/tirecalculator.html">http://ctny.audiworld.com/mark/a4/calculator/tirecalculator.html</a</li></ul>

Its not gonna help performance-just ruin the ride & damage expensive rims.Tire choices are slim
 

in the larger sizes. Unless you live near glass-smooth roads, I would not reccommend anything over 17. The weight of the larger wheel+tire combo will slow you down in accelerating. braking, and roadholding. For some reason, the oversize wheels are popular with people who want their car to LOOK fast. LOL

gas consumption a bit higher due to higher weight and larger contact patch.

my stock comes with 205/55 R16
 

is that enough?
im thinking the avus 17" with 225/45s look so much fiercer, just don't want to compromise performance by a lot.
any suggestions?

I think you mean 215/55-16.

my car is 2nd fastest. CVTMAN has chrome. If you go big get forged

Avus 225/45R17s are fine. One of my favorite OEM wheels.
 

And of course you will affect your car's overall performance. But in the case of going to 17" it varies. The car will handle differently when you change. I've driven cars with your tire size, and to me the car feels bouncy and more sloppy.

Chrome? Yuck. It weakens the aluminium. Forged is good, but wheel damage is always an issue.

Not only do tire choices decrease, the cost per tire starts to increase significantly as well.