Karger's confusing acceleration with velocity. Because of the law of conservation of momentum, you will slow down if you increase your mass while moving. If your mass doubles, your speed will half. However, in this situation involving Lashings, that would not come into play because you are not changing mass while moving. A Lashing defines an acceleration, not dependant on the mass off the object. The mass only influences the amount of Stormlight it takes. If you were to Lash yourself while tapping an ironmind, you would accelerate as normal, and you would have, as you suggested, a large amount of energy. Sadly, because of the need for stormlight as a fuel, this would still not work as you hope.
Let's say 1 Lashing gives 10 m/s^2 of acceleration (about Earth gravitational acceleration). Let's also say you have enough Stormlight to Lash your mass (50 kg) once for 1 s. Without tapping the ironmind, you will be travelling at 10 m/s after that second and have 2500 J of kinetic energy. If you tapped your ironmind enough to double your mass (to 100 kg), you would then only have enough Stormlight for .5 s of a single Lashing. After that time, you would be going at 5 m/s and have 1250 J of kinetic energy, which is less than before. Let's say you were filling the ironmind instead for a mass of 25 kg. Your stormlight would last 2 s, after which you would be going at 20 m/s and have 5000 J of kinetic energy. In all three of these situations, your momentum is the same, 500 kg m/s. So, in a vacuum, in terms of Stormlight efficiency, you're better off storing weight if you want as much energy as possible. Another factor to consider is that outside of a vacuum, decreasing your mass also decreases your terminal velocity. Also, even in close quarters, you're better off using a double lashing than doubling your mass, since both will take double the Stormlight, but a double lashing means twice the end velocity, which is more important for kinetic energy (half of mass times velocity squared).