I heard it noted in other threads that spell penetration is essentially a useless stat for pve and all bosses have a built in irreducible amount of spell resist, however I couldn't find the thread for that (I'm sorry this thread may not deserve its own topic. was unable to find the topic I was looking for with the search feature, may have missed it, but looked at hard as I possibly could.).

Could anyone tell me A) where to find said thread with data backing up said claims and B) how I can start collecting my own data about this, so I can start testing this out on bosses to satisfy my own curiosity or perhaps even add some more significant data.

Some day ago I did the same search (I wanted to know what is the effect of Spell Penetration). I found those threads and read them. The point with Spell Penetration, as long as I've understood, it's that it decrease the resistance provided by gear. This means that while your pvp enemies try to gear up for shadow res (i.e.) you can gear up spell penetration and negate them their bonus shadow res.

Bosses only have level based spell resistance, according to the various threads, and this means that spell penetration does nothing, because it's intended to counter gear based resistance.

Thanks, I know this topic probably didn't derserve its own thread, I said that, but I was having trouble finding it, thanks for those 2 thread links, they both contained links to:
TBC Mage Theorycrafting

Which was the right thread most likely, I just didn't skim through all 100+ pages when I had looked at it previously.

Yet you created it anyway. I hope it was worth it.