What sort of power are you looking to achieve?
anything over stage 1 you are probably going to need to look at a limited slip differential for a diesel because of the power delivery, if not a different ECU to manage power control as the OEM is not designed beyond stage 1 without other additions to manage the power delivery.
I'm not exactly experienced with tuning audis but my previous Passat 2.0 TDI 2012 (144bhp) was tuned to 281bhp. to get that sort of punch beyond 30-40bhp (stage 1) you needed to consider a lot more components than you would typically do with a petrol (which is why petrol engines are preferred for competitive racing/tuning).
to name a few...
- larger/performance intercooler (lower air and engine temps always safer to work with when tuning)
- air intake/filter (more air, cooler temps, more compression when added with more fuel!)
- bigger injectors (more fuel, more power, you will likely need a uprated fuel pump to suit).
- Turbo modifications (hybrid turbo) im aware that the c8 model has E-Turbo (electric turbo) i'm not aware of any benefits with tuning, in many cases tuners typically opt to remove in favour of a more conventional hybrid turbo given its more flexible tuning parameters.
- Exhaust modifications, typically on any car with ad-blue is now considered a substantial overhaul when you start mucking about with anything beyond the manifold. Its benefits in gains are limited by the ad-blue system that needs to be mapped out. there is arguments in doing so will make the car illegal because of the emission requirement. I have been talked out of this many times on previous cars that use the system. equally it is by far the largest contributing factor preventing any big performance numbers compared to non-ad blue equivariant systems.
- Limited slip differential(s), (specific to VAG automatics: gearbox tuning) if you have a Quattro be prepared for £1000+ invoice!