I'll reiterate...measuring the time it takes between the tank turning on to reheat the water after no useage and knowing the amount and delta between when the thermostat turns off and back on again will let you calculate the power usage and therefore the heat loss. if you do this now before the incoming water gets really cold and then again sometime say in February, you should have a good data point. If you can wait long enough, do it again in say August after everything has heat-soaked. You could probably hook something up to tell you how often the thing ran by wiring in a clock circuit for cheap, although you would need to be very careful how you did it to remain safe. There are plug-in watt meters (not applicable here), and probably a wired in version that might be less expensive than using a meter. But, other than interesting, adding insulation is by far less expensive.
Some spec sheets include info on the insulation R-factor. If not, maybe an e-mail or call to the tech support of the manufacturer might give you the info you desire without any further effort.