But hey, wouldnt that mean that the high voltage cathode would be overpowered and overheat on European RS ballast ?
Cause that is not the case, it is barely visible dim red
The filaments are designed to be preheated by about 1.5x the nominal arc current (that is, what the preheat ballasts usually deliver into a short circuit; it is assumed to be time limited, the "unlimited" rating is then about 1.3x the ballast rating). That yields the 9V rating during preheat phase.
In European RS ballasts (a.k.a. SRS) the heating current disappears once the arc ignites (it is the way, how the current phase shifts work in that circuit). But due to the topology, the preheating current is only what flows through the capacitor, so a bit less than the rated arc current. It is sufficient to reach at least some emission, so to allow an ignition, but then the filaments are supposed to completely warm up from the arc by itself. And that is, why you see the cathodes only dim red - the current is just not sufficient for a full warmup.
When the lamp is burning, only part of the filament (in a preheat circuit, where the current comes just from one side) is biased by the current and that current is what the ballast feeds. But that means way less than 50% of the power the filament needs to remain at the emission temperature, the rest comes from the cathode fall (in fact the energy of the ions hitting the cathode). In SRS the current emerges practically from both sides of the filament, so the only heat source is the arc cathode fall itself.
Normally the cathode fall responds to the cathode emission, hence the cathode temperature and that mechanism maintains the temperature exactly on the desired level (lower emission -> higher fall -> faster ions -> higher power dissipation -> the cathode heats up). The thing is then designed so, the resulting cathode fall is low enough to prevent sputtering, but still high enough so that mechanism has enough room to correct variations (so the filament would not overheat).
And this mechanism then settles on higher cathode fall, when the arc current becomes smaller.
And that will happen when a 0.43A rated tube gets operated from a 0.25A ballast without sufficient auxiliary electrode heating.
On the other hand that difference is not that huge (there is yet another effect: The heat is being lost by the emitted electrons not returning back, so higher current means more heat removed from the cathode), so the cathode drop will increase from about 15V to less than 25V or so, what is still not that excessive as e.g. cold starts (there we are talking about above 100V or so).