What is the noise floor of the 16 bit recorder? I.e. if you do a "silent" recording with all gains at minimum, where does the noise sit? -85? -90dB? The theoretical dynamic range is 96dB so if you can get around -90dBfs you are doing pretty well and it is very unlikely that the sources you use and the mic pre amps are that good? Even if the pres were that quiet it is likely you could not get the room quiet enough to take advantage of the fact?
So, bottom line, even if you kept to the -18dB fs level of 24 bits it is likely the converters would be quieter than everything else.
Dave.