Ive done alot of reading into it, and alot of the bigger mics are the exact same circuitry!
Other than replacing components, the only mod to the circuit itself would be to use a cathode follower circuit for the tube in place of what they usually do on the cheaper mics (which i cant remember what its called atm). A cathode follower should give a nicer sound, and much more headroom

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The only problem there could be would be any possibly resonances within the mic casing, but i think that may be more of a problem with ribbon mics?
Here is an example of the differences between a $200 and $1200 mic:
R-F-T Telefunken M16 and Apex 460 Bench Test
They stated further on that the mic that they sent out was a prototype, which, IMO they only did because they got caught out!