Best would be to change the pistons for proper 8.0:1 ones.
Alternatives:
https://www.bar-tek.com/de/1.8t-dicke-metall-zylinderkopfdichtung-bar-tekOf course they don't say how much it drops CR. Height is quoted as 1.92mm.
https://www.turbozentrum.de/Dicke-Zylinderkopfdichtung-VAG-18T-MetallkopfdichtungThey say 0.5-1.0 and the price is very good. Height is quoted as 2.35mm.
https://www.034motorsport.com/head-gasket-compression-dropping-multi-layer-steel-20v-1-8t.htmlDrops 0.5. Height not quoted.
Here's a website that uses a plate and they let you customize it:
https://www.turbo-concepts.de/motorenbau/audi-vw-seat-skoda/1.8t/rumpfmotor/verdichtungsreduzierung/149/verdichtungsreduzierplatte/spezial-kopfdichtung-1.8t-20vAnd here is another one, that is cheaper:
https://www.lspeed-racing.de/product_info.php?info=p1603_audi-1-8t-20v-zylinderkopfdichtung-mit-verdichtungsreduzierung.htmlI would just get the thickest one. The only thing the compression ratio is beneficial for on a turbocharged car on pump fuel is fuel economy, at least as long as it is higher than 7 or so.
So maybe you will lose 0.1L/100km, but you will easily and safer make more power. Which is the point I think?!
If your CR is now closer to 9.5 then the thickest variant will drop it around 8-8.2. I recommend using ARP studs with the proper torque too...
Alternative is the Turbozentrum multilayer, but the effect will not be that big. You probably need around 0.1-0.3mm extra thickness just to get back to your original CR. It depends how badly your head was warped when it was skimmed. And the Turbozentrum one gives you about 0.8mm over stock.
Keep in mind that whole timing map will need re-tuning on dyno, not just WOT. If you do not take advantage of the CR then the car will be slower off boost.
Also the 9.0:1 CR is ok for a standard K04, not for a turbo that can give 1.5x the output. The 9.0:1 is already a tradeoff between fuel efficiency and performance/reliability from factory.
You should be aiming below 8.5.
VAG dropped the CR by 0.5 going from 180hp to 225hp, which is only a 25% power increase.