I would be interested in seeing those studies if you have any links. The only studies I've seen on it are totally bogus. I remember on 2 studies they came to the conclusion that pre exhaust doesn't work because once you hit the compound movement your secondary muscles are activated more than the primary muscle. The study didn't make any sense though because that is technically what you want. You want your pecs to reach early failure on the bench press after a pec fly, which means your triceps end up with more stimulation.
I know the Pec deck and chest press study that you are talking about. Activation in the target muscle (pecs) was no greater in the pre fatigued muscles in the chest press. So doing a fly beforehand did not help activation in the pecs which is what we are trying to achieve.
(I can't link the studies not enough posts)
Gentil P1, Oliveira E, de Araújo Rocha Júnior V, do Carmo J, Bottaro M. J Strength Cond Res. 2007
Years later interpreted that research the same way. This is a review on exercise order in general
Simão R, et al. Sports Med. 2012
This is probably one of the other ones you were talking about pre exhausting quads leg extension and then doing leg press after.
Augustsson J1, Thomeé R, Hörnstedt P, Lindblom J, Karlsson J, Grimby G J Strength Cond Res. 2003
"The activation of the rectus femoris and vastus lateralis muscles during the leg press exercise was significantly less when subjects were pre exhausted."
So again the activation of the pre exhausted muscles were less on the compound movement when target muscles were pre exhausted..
Not only that but in this study the researchers noted that "in a pre-exhausted state the subjects performed significantly less repetitions of the leg press exercise" as I stated before it would surely cause a dip in performance and thus a decrease in volume.
If pre exhaustion will decrease total training volume it will definitely decrease results as volume is considered by researchers to be one of, if not the most important variable in training for size.