I've learned that bird shot won't penetrate enough in most situations to mortally wound an assailant.
What you've learned is basically false. Facts are that most of the time bird shot will mortally wound a BG hit in the chest at HD distance. Is it the best? I wouldn't say that but you're right, 1 oz of lead at 1200 feet per second is sure to leave a mark whether it be in one solid chunk or 400.
Where these threads go completely sideways is SOME try to lump all bird shot into one group and buckshot into another. They compare 00 to some lame trap load. Bird shot generally goes from #2 thru 7 1/2 or 8. You also have BB, T, F and bunch of other shot available. There's a big flippin difference between all of them. Personally I see no reason to go any bigger than 4 buck and anything thing lead or hevishot down to maybe #4 shot is bound to save the day with dramatic results. Like I said, big difference between those I mentioned and 7 1/2 dove loads.
For the record, the plastic wad will blow thru plywood more times than not from 15 feet.