I have never owned the Inspire, but I’ve owned three Bottlehead Kaiju amplifiers in my time. I have also owned more expensive SET amps, and I will say that at their price point, when modded, they are very difficult to beat.
Since the Kaiju simply sits on the chassis, if you plan to buy it used then it’s easy to mod as everything is accessible underneath. In my case I upgraded input coupling caps and signal caps with a number of different brands, but my favorites were Mundorf Silver Gold Oil capacitors and Audyn True Copper Max. Th Mundorf specifically added more dynamics, transparency and detail well beyond what a typical 300B SET amp does.
the performance of the amplifier was deserving of the best tubes. I enjoyed both the EML 300B Mesh and Psvane ACME 300B as my favorites, along with six other pairs or so that I tried that didn’t perform as well (Sophia Royal Princess, Gold Lion, EH Gold Pin, Psvane TII, Elrog, Shuguang Black Treasure).
The transformers on the Kaiju are specifically designed for the amplifier, so there is no real benefit in trying to upgrade to fancier ones like Hashimoto or Tango.
I hope this is helpful if you do decide to go with the Kaiju down the road.