Unfortunately that is probably a useless number written to help sell the item! That efficiency can only be achieved in an ideal situation: a certain input and output voltage and current. In all other situations, almost certainly including your project, the efficiency will be less than that. They do not state a minimum efficiency, I suspect!
You could measure the actual efficiency, once you have your project working. With a steady, non-flashing pattern on the LEDs, at different brightness levels, use your DMM to measure the power entering the converter (amps X volts) and the output power. That would be interesting to share with the forum.
Do not be too concerned about the current drawn by the strip. That 7.5A is with all LEDs on max brightness white. Other colours, flashing patterns and lower brightness will reduce that current down to a fraction of that figure. Full brightness green, for example will only be 33% of the 7.5A.