In theory how fast could nerve signals travel if the nerve fibre was perfectly insulated?
For some background on saltatory conduction: https://en.wikipedia.org/wiki/Saltatory_conduction

I feel that a "perfectly insulated" neuron couldn't actually allow the signal to travel. Insulation (which, in neurons, is basically layers of fat wrapped around axons so ions cannot pass) allows the electrical signal to move further along axons (less ionic leakage) and faster (due to decreased membrane capacitance-it takes less charge movement to generate the same potential difference across the membrane; see this question: How is membrane capacitance related to the increased speed of saltatory conduction?)

Also, you should check out wikipedia's page on cable theory of neurons:
https://en.wikipedia.org/wiki/Cable_theory

This is why myelination increases conduction speed, as you may or may not already know. The increased length of the ionic signal (due to the inhibited leakage) allows voltage-gated ion channels which are further away to be activated as a result of any given local membrane depolarization. This all happens faster due to the lower capacitance. 

However, the only way the action potential can actually "travel" at all is due to the sequential activation of voltage-gated ion channels along the axon. This is the case in both myelinated and unmyelinated axons, it's just that the spacing is greater when myelinated. In the insulated, fat-wrapped regions of axonal membrane, there are no ion channels, voltage-gated or otherwise. If there were, the layers of fat would not allow ions to pass through anyway at this point. Thus, the "perfectly insulated" axon could never actually propagate a signal, because the signal still requires the chain reaction of voltage-gated ion channel activation and sodium influx at some point. Myelinated conduction is faster because the spacing is increased, but increasing the spacing to infinity would not allow the action potential to be passed on.

I suspect that there is a certain threshold in the extent of myelination where the spacing between nodes of Ranvier becomes to great to have effective propagation of action potentials. I wouldn't be surprised if evolution has resulted i a relatively optimal spacing of nodes. I suppose a better question might be: what is the speed of conduction between nodes?

I suppose it would be possible to send current through a perfectly insulated axon and potentially activate a voltage gated calcium channel at the terminal. However, This would be different than an action potential and would be effectively reducing the neuron to an electrical wire. Sometimes it's better to keep things simple.