Hi Gas Mantle,
That is a beautiful little boiler and your cladding looks excellent. Doing the top cap as well will not improve the performance any but will reduce the chance of burnt fingers due to the accidental touch, so well worth while.
I understand your feeling that it is hard to tell on the face of it how much it has helped, however consider this.
Hard to do much with the heat up time apart from repeating the experiment a few times to see if it is consistent, or at least so you can take an average. This is because you are heating up the copper shell, fire box metal and even the timber cladding, hard to quantify without a lot of measurements.
But at 30 psi the water boils at 133 deg C (271 F) while at 60 psi it boils at 152 deg C (305 F). If the ambient temperature was at 20 deg C (68 F), the heat loss would be about 20% more at the higher temperature without your cladding. With your cladding you have maintained pressure despite 20% more heat loss, a significant improvement. This is based on the temperature difference assuming a constant heat transfer coefficient. However convection loss would be more vigorous with the higher temperature difference so the actual saving would be higher. Steady temperature means the shell etc is no longer relevant.
I am assuming you throttled the steam at the boiler outlet when running at the higher pressure, otherwise the engine would be running a lot faster, or driving a bigger load to keep the revs about the same. Running throttled means that the energy consumption of the engine is only just slightly higher at the higher pressure, due to slightly lower efficiency, but you have maintained this despite the higher boiler temperature which translates into being able to drive a higher load or run longer from the same amount of fuel.
If you put a second layer of planks, you might get a further say 15% reduction in heat loss, getting to be a serious improvement.
An excellent job and an excellent result.