Bike fit / correct riding position

You're right. I did have another link but didn't post. It's a few mm according to this.

https://forum.bikeradar.com/discussion/12891402/saddle-height-between-spd-and-spd-sl-pedals

I'm sure I've looked into this before, maybe even posted differences on here.

Speedplay:
https://www.lfgss.com/comments/14332578/

Time ATAC:
https://www.lfgss.com/comments/6781015/

SPDs:
https://www.lfgss.com/comments/8159952/

https://outerask.com/spd-vs-spd-sl-stack-height/
"Shimano’s XTR pedal stack up in terms of stack height? Well, it’s one of the lowest Profile pedals on the market with a mere 10mm of stack height. "

"Shimano Spd M520 Stack Height The Shimano SPD-M520 is a mountain bike pedal that was introduced in 1992. It features a stack height of 14mm"

"The M540 has a stack height of 12.7mm"

"The average stack height for SPD pedals is 14.5mm, while the average stack height for SPD-SL pedals is 9.5mm."

https://forums.thepaceline.net/showthread.php?p=3025620
One source reports that the XT XC pedals have a stack height of 18mm.
https://mtbgearbox.com/shimano-xt-vs-xtr-pedals/
That’s versus 13.7mm for Ultegra. XTR and DA are lower.
Then you’d need to consider the stack height of the shoe, e.g. the sole thickness.

Road pedal stack heights:
https://weightweenies.starbike.com/forum/viewtopic.php?t=164853

R8000: "Shimano has decreased the pedal’s stack height by 0.7mm"
https://www.cyclingweekly.com/reviews/pedals/shimano-ultegra-pedals

https://www.bikeforums.net/18470444-post1.html
"On the web I found a source that said 13.7mm for the Ultegra pedals (did not mention though if it's the 6800 or an earlier model) and 14.3mm for a Ritchey one-sided SPD road pedal which is the closest thing I could find to the PD-A600. Double sided Shimano SPD pedals are listed as 17mm. Then I guess the shoes play a role as well."