I would love to get an answer from Mr Friedman...
I have heard the Friedman Plexi mod is same/similar to what he did for Steve Stevens. Since Steve used a metal face '70's (pre 2203 JMP I believe, so NMV.. I *think* it is an early/mid '70's 1959 SL), I would *assume* the mod was created to emulate that. But, it may be simpler, more complex.. well, only Dave or someone whom he has discussed it with can say.
The Brahma OTOH, was supposed to be based upon Lynch's '68 Plexi. The '68 Plexi is also quite famous for it's use by EVH. There is a vid on Youtube of Dave working on a proto Brahma, mod'd, with George. If you check ebay (and other places), once in a very great while a "Brahma Friedman Corrected" will appear. Supposedly these are Brahma's that Dave has tweaked to be (closer to) Lynch's current Brahma being used.
If all the above is true, then I would say they are very different animals. From '68 to '69, the panels went from plexi to metal, and the transformers got spread apart (changes the sound, subtly), caps to top, etc.
I wrote all that so that I can learn as well... I want a Dave mod'd Plexi, and would love to try one done to Steve's specs. Would love to play that Brahma proto George used in the vid (no boost? just guitar into module?) I wish they would have said what speakers were in that cab... Hopefully someone has spoken to Dave and knows the full scoop
Oh- edit to add- Rig Talk has now added Suhr and.. Rack Systems (Dave Friedman) to their stable. I posted about this there as I am very curious.. these are two modules I am VERY interested in.