IEC 60076-5:2000 Power transformers - Part 5: Ability to withstand short circuit recommends a minimum impedance for transformers.
Unlike other standards such as the IEEE or AS which recommends typical values, IEC recommends minimum values. As these are minimum values, it does not mean that as an electrical designer, you can select very high impedances for your transformers. For example, IEC recommend 4%Z minimum impedance for a 300 kVA transformer. It does not mean however that you could have a 6.5%Z. Yes you can, there will be nobody preventing you to do that but transformer %Z in directly proportional to the transformer cost, size, and weight.
Why do you have to pay more for a transformer when you can buy it cheaper b reducing the %Z? The only time you will select a higher impedance for your transformer will be to reduced through-fault current to the transformer.