There’s quite a few PvP builds that run a 5 plant to save weight.
My bi-ree Lance has one, and I’m running a bunch of LR beams and rails. I think it’s a size 5 guardian plant vs G4 overcharged 5.
There are, mine included, however the tradeoff for running a OC powerplant is heat, which is a comprimise that almost forces less ToT due to the decreased efficiency, amost forcing a kinetic loadout, in turn making the damage output much more restrained and also forcing it to be governed by resistence based multipliers. Though I gather your build includes a Thermal Vent beam which is an effective counter to heat however it is still governed by resistance values thus further reducing the damage output of the ship, again a tradeoff for the high power Railguns. (Also governed by resistance damage)
As for counters vs shield vs hull..
Hull counters are more numerous by a vast margin.. Scramble, Emmisives, Missiles, Packhounds, Railguns, High Yeild (Even post nerf this still cakes modules SUPER fast), Corrosive and the fact that hull damage is permanent.
Shields regen, have what are essentially the option of health potions, and have three counters, Feedback (Which feels like it doesn't work effectively enough in the current build), Reverb Torpedos and Reverb Mines, both of which you are obscenely lucky to effectively strip a shield generator with, in fact against any experienced pilot you are more likely to screw yourself over than your opponent be it through having the Torpedos led round into your thrusters or just straight up wasting hardpoints.
Whilst I agree that HP overinflation is a huge problem it is somewhat justifyable for a large vessel to have near inpenetrable shielding, thats kind of the point of them. However a medium vessel that already has arguably the best hardpoint options, a boatload of speed and manuverability has no business having the capability to run shielding that is more comparable to the large ships than anything in its own weight class.
A C5 powerplant would go a very long way to stop the ridiculous bank tanks that seem to be the most popular build for the vessel and would allow other ships such as the FAS and Chieftan to sit much more comfortably within the meta.
Bi-Weave FDL pilots are often the ones getting the most out of the ship through pure skill, however the Prismatic vessels allow any pilot with any skill level to survive for a much longer time period than they should by al rights be able to. In a PvP environment the defining characteristic should be skill and skill alone, not a build that any monkey can throw together in five minutes in coriolis and get a good result out of.