A couple of years ago, I contacted David Allen to obtain just a middle PUP for my Jerry-ified '99 USA Reverend Avenger. He wouldn't sell me one. He told me the PUP wouldn't work well except as within a full set of 3. This, to me, was nonsense. We mix and match PUPs all the time because we are all of us unique and not necessarily chasing the same exact specs and tones and the PUPs typically work fine for us that way. Plus they don't interact one to the next when playing just through one. However, it's Allen's business and if that's how he prefers to run it, so be it.
So, instead, I tested all 3 of the stock Kent Armstrong s-style PUPs from the Avenger in the middle position. The 6.0K bridge PUP sounded wonderful so I kept it there. When I A-B'd the guitar with my Scarlet Fire Wolf (DA's) middle position PUP, sure, the tones differ some (as would be expected, due to the two guitars' very different construction, even if the two PUPs actually did sound identical), but both are, for me, effective for playing Dead.
The bottom line for me is that, in my view, there are probably many middle Strat PUP options that could work. Indeed, I find the split middle Super II on my Philtone Jerrycaster more different sounding from both my Scarlet Fire Wolf's and Reverend Avenger's middle PUPs than either of the latter are to each other, yet all three are fine options for playing Dead.
The Avenger's bridge PUP is a 2-wire Dimarzio Super Distortion I bought new in college c. 1976-7; middle is the 6.0K Kent Armstrong strat PUP that was the Avenger's stock bridge PUP; neck PUP is a 9.0K Fralin Steel Pole 43 tapped at 6.0K. 5 way switch yields: bridge PUP alone; bridge and middle PUPs in parallel; middle PUP alone; neck PUP tapped; full neck PUP.