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.

throw std::ios_base::failure("Unknown transaction optional data");
        }
    } else {
        
        assert(tx.wit.vtxinwit.size() <= tx.vin.size());
        if (fAllowWitness) {
            
            if (!tx.wit.IsNull()) {
                flags |= 1;
            }
        }
        if (flags) {
            
            std::vector<CTxIn> vinDummy;
            READWRITE(vinDummy);
            READWRITE(flags);
        }
        READWRITE(*const_cast<std::vector<CTxIn>*>(&tx.vin));
        READWRITE(*const_cast<std::vector<CTxOut>*>(&tx.vout));
        if (flags & 1) {
            const_cast<CTxWitness*>(&tx.wit)->vtxinwit.resize(tx.vin.size());
            READWRITE(tx.wit);
        }
    }
    READWRITE(*const_cast<uint32_t*>(&tx.nLockTime));
}
/** The basic transaction that is broadcasted on the network and contained in
 * blocks.  A transaction can contain multiple inputs and outputs.
 */
class CTransaction