TC,
You are right. Fuel flow meters work via an impellor in the fuel stream i.e. they actually measure volume flow so if the SG differs they will not know the difference in in weight. Some fuel contents gauges (electrical capacitance type for certain) also tell porkies if the SG is different to that at which they were calibrated.
If a fuel weighs less and has better calorific value it must be a better fuel. Less energy will be expended carrying it and the fuel flow required to get the same power output will be less, although there may be T4 limiting problems requiring engine adjustment. I remember years ago when our sqn went to RAFG the engineers had hours of fun resetting the fuel flows due to the fuel difference.