The whole replicator-based reactor thing is indeed intended for end-game renewable power generation, but in general I don't think reactors are poorly efficient.
; Balance Values for Fermenter
[balance / fermenter]
need_amount_biomass_per_run = 20
output_amount_biogas_per_run = 400
hU_per_run = 4000
biomass_per_fertilizier = 500
To run a biomass-based fluid reactor that produces 20,000 HU/s (which is identical to what you mentioned and convenient to calculate), it needs 10 fermenters to consume all the heat at 100 HU/t each, which produces 4,000 mB Biogas in 2 seconds, which needs 200 semifluid generators to burn all of those biogas, as they burn 10 mB biogas every 1 second. In total you get 3200 EU/t.
The design that I used in my MOX array was a 4-chamber design with a single quad MOX rod and 4 iridium reflectors, running at roughly 82% heat and giving it a raw output of 600 EU/t each. Taking replication costs into account it outputs 427.668 EU/t on average. A 20-reactor array, which produces 8553.36 EU/t on average, has a volume of 12x11x35 = 4620 blocks. If a 20,000 HU/s design is more space efficient, the volume of it must be under 1728.44 blocks, which to me looks difficult.
Another thing is that your design doesn't scale up very well. I designed my reactors to be easily stackable, but if you want to, for instance, double you output you literally have to build everything again though some parts might be similar. Without overclockers in my fuel processing section, I would consider the thermal centrifuge to be the bottleneck of the maximum rate of fuel production, taking 25s to process a single item, thus taking 50s to process a single uranium ore. Replicating enough uranium ore (without scrap) to match this speed requires an input of 16,070 EU/t on average. Considering only 2872s/10000s of the power produced is used to replicate uranium, this would mean that it can take in 55,950 EU/t for the amount of time that EU is used for uranium replication, which equals to 93.25 reactors running simultaneously. If there are 90, they would have an average output of 38490 EU/t and a volume of 12x11x105 = 13860 blocks. If the 20,000 HU/s design is used then its individual volume should be no more than 1152 blocks.
In terms of building efforts your design would be tedious to build on a large scale, while mine should be much simpler.
As for the amount of machine blocks needed, my 20-reactor design needs 31 + 20 + 80 = 131 machines, while only one of yours needs 200+.
Also, don't forget lag, which is the most basic limiting factor of how large anything can be built. The more machines you use in a single build, the laggier it is, and in turn less of it can be built.
I currently have no idea about what your design is like, so that's what I can say for now. It would be great help for me to analyze exactly how much more efficient your setup is if you could post a world download.