It's an entirely valid thing to do.

In Fred Dibnahs day, yes.

It is overspecced for five main reasons.

a) I intend to install an additional shower room at some point.

b) By not taking into account boiler output, it is useful when run from the
immersion, as it is at present.

c) If the calculations had been wrong (perhaps over optimistic from
manufacturer) it would cost a bomb to sort out.

d) It makes any destratification of the temperature profile less important.

e) It does not significantly impact on the space used or the purchase price.

Christian.