The problem with your theory is that it requires there to be a 054B that originally did NOT have IEP that subsequently displaced several hundred tons more WITH IEP that was then 'fixed' with a redesign. This to me is entirely NOT the most reasonable assumption as IEP has been associated with the 054B for a long time. Though it is within the realm of possibility, it is obviously not the default assumption.
I fully agree that IEPS has always been associated with 054B for a long time. But I don't see how my idea suggests that isn't the case.
Given what is written, clearly he is saying that originally they thought 054B would have to displace a few hundred tons more (but relative to what?) due to the introduction of IEPS (well technically he writes "改系统" which really makes "changing system" which I we can sensibly interpret to changing or advancing new subsystem/s) -- however additional design allowed them to prevent the increase of that few hundred tons.
Putting it very simply:
054B (with IEPS) original design: added a few hundred extra tons relative to XYZ
054B (with IEPS) revised design: did not add those few hundred extra tons that 054B-original-design would've had relative to XYZ, while still having the same systems (including IEPS) that the 054B-original-design would've had.
In other words, both the "original" 054B and the "current" 054B both included IEPS, but design changes meant the "current" 054B managed to include IEPS without the increase in extra few hundred tons displacement of the "original" 054B which also included IEPS. The question is what the "few hundred tons" greater displacement was relative to -- was it relative to 054A, or relative to 054B now, and if it is relative to the latter, well that means we don't know
.... that said, I'm a little bit dubious as to whether fzgfzy is actually referring to IEPS when he says "改系统" because 系统 can refer to systems or subsystems in a general sense rather than explicitly IEPS, unless one is interpreting it in context of his overall post which talks about IEPS a bit more. If he is not referring to IEPS explicitly, then he may be talking about any number of changed "systems" or "subsystems" when he is talking about the "few hundred tons".