How much leakage it has right now is not that important. What's important is whether it can change anytime with no advance warning, generally the answer is yes, it can. That's where the transformer will provide your safety
(as long as it is intact - which is worth testing, but unlike a soldering iron, an isolation transformer has huge safety margins in its isolation materials, and they are not stressed by high temperatures)
When you use equipment like a standard user uses home appliances (not covers off and sticking hands in), as long as your equipment is in good condition and reliably grounded (including the ground wiring throughout the house being intact), its perfectly safe as is and does not require any additional isolation between you and ground
Isolation by transformer provides the same level of safety as ground or higher. Electrical codes do define some dangerous situations (typically in the context of industrial settings, the cases where you are with a line powered tool in a confined metal duct ect) where grounding is not considered good enough, but isolation by transformer is - for a reason
I am always barefoot as well, except when actually dealing with electricity or with the electrical installation
Workstations have also a relation to this :
A grounded iron is safe to use, but not ESD-safe in all cases (to prevent damage to components) :
- The source of the static charge may be present in the circuit, and discharge to the iron. The damage will be exactly the same as if the source was in the iron and discharge to the circuit
- The most sensitive components nowadays may have abs max ratings in the order of single volts or less. If the circuit is grounded from one source (you and te floor below) and the iron from another source (line ground) the voltage difference between them alone may be sufficient to damage something
So to improve its ESD safety, an iron is better be electrically isolated, than low-resistance grounded. Then it is still grounded through some high value resistance (typ 1MOhm), to prevent slow static charge build up if a static source is present
Then for electrical safety, the iron is powered by extra low voltage (<50V), and the transformer is contained inside the station box
Not all workstation irons are like that, and some can be reconfigured between different grounding settings. The low end ones and those where the controller is in the handle are often powered directly by line voltage and must be low resistance grounded