Hello Gareth.
Let me start by saying I have no knowledge of the differences between cast (or would they actually be forged?) and pressed arms, and no experience of changing them on a A3.
I am unclear of your reason to change to cast arms. Is it a matter of cost, durability, availability, strength or something else? If the current pressed arms are the originals they have lasted 15 years, and I imagine pressed replacements would last more or less as long, so durability is not a problem unless you plan to keep the car running past the age of 30. In your position I would fit whatever was available and known to fit, in the expectation that they would last the rest of the useful life of the car. If that means unbranded pressed steel parts so be it.
Cliff.