The user of-course can already implicitly work out this information from comparing the fee-rate with the actual fee.

I think that it would be useful for people not wanting to do this arithmetic in their head.

{
    nTxSize = CalculateModifiedSize(nTxSize);
    if (nTxSize == 0) return 0.0;
    return dPriorityInputs / nTxSize;
}
unsigned int CTransaction::CalculateModifiedSize(unsigned int nTxSize) const
{
    
    
    
    
    
    if (nTxSize == 0)
        nTxSize = (GetTransactionWeight(*this) + WITNESS_SCALE_FACTOR - 1) / WITNESS_SCALE_FACTOR;
    for (std::vector<CTxIn>::const_iterator it(vin.begin()); it != vin.end(); ++it)
    {
        unsigned int offset = 41U + std::min(110U, (unsigned int)it->scriptSig.size());
        if (nTxSize > offset)
            nTxSize -= offset;
    }
    return nTxSize;
}
unsigned int CTransaction::GetTotalSize() const