It depends. There is the cost of REF protection and the cost of fixing any damage caused by earth faults. For large transformers REF is worth the price for the protection it gives. For small transformers, it is not.
At 1500 kVA, it is debatable. I know engineers (probably including myself) who would consider the use of REF. I also know a lot of engineers who would not use it on that size of transformer.
I'm not familiar with the Shell standards, but if they are your client I would tend to follow what they say and not apply it. You could always run it past the clients engineer.