Measuring Current, Resistance and power capacity of an Incandescent bulb
You are correct to trust V * I to measure power. As pointed out by Whit3rd, lamp resistance varies with temperature in a non-linear way. You should take care that "V" and "I" measurements are taken within a short time-frame, lest lamp resistance changes between these two measurements.
Heating and cooling of lamp filaments can also create some nasty dynamic effects that your power source may have difficulty dealing with. If your power source is intended to power incandescent lamps, then your load test is realistic.