Fate/Zero Volume 3 Light Novel, Page 98, during the fight with Gilles' squid monster
"Even with the exceptionally powerful Noble Phantasms possessed by Rider and Archer, it will not be enough to fell this monster. No matter how large a force they use to trample it,
it is meaningless if all its injuries can be regenerated from instantly.
To defeat this horror, one must simply deliver a strike that covers it entirely, obliterating it down to the last scrap of flesh - what is needed is not an Anti-Army, but an Anti-Fortress Noble Phantasm.
Excalibur could achieve this, but Saber could not use it now. The fatal secret technique
which releases in a single blow an enormous surge of energy rivalling her entire prana supply; no matter the circumstance, for her to use it, she must swing the sword with both hands."
The reason why only Saber could defeat Gilles' monster was because non of the other Servants there possessed a Noble Phantasms that covered a large enough area to engulf the entirety of the monster. The second that someone like Rider made a dent in the monster, the injury would just instantly heal itself. Also, Excalibur is stated to release power that rivals Saber's entire supply of magical energy, so clearly the difference between Servants and their NPs aren't millions of times.
Unless Kanshou and Bakuya became a beam saber, and single strike means ray of death, I think Nasu doesn´t agree with you
I mean, we've never seen the "original Kansho and Bakuya" (as the question phrases it) so who knows how they actually work. Considering how many Saber-class Servants have beam based swords, it wouldn't surprise me that the "true" form of Kansho and Bakuya can do something similar. Nasu also specifies that their true NP form would be "extremely strong anti-monster Noble Phantasms". The Anti-Monster part may give them a conceptual advantage against Gilles' monster allowing them to negate his regeneration, or something similar.
The difference in power has never been even alluded to
Hasn't it been stated that A/A+ rank physical attacks = C rank Noble Phantasms. Can't we work out a difference based on that?