The only thing we know for sure is that in order to hit 96% RTP buys have to pay an average of 86.4x back. Since you go straight into a 12 spin bonus every time with a starting win of 0 that has to mean all 86.4x is coming from those 12 spins (+ any retriggers).
Now again I don't know this but this is what people say. Bonanza bonuses are not predetermined but each spin is random. Assuming that is accurate then yes
BTG could use different weighted reels for their buys vs spun in bonuses. But it seems more likely they are the same reels.
EG: Natural GOLD with no tumble start with 0 win, 12 spins. That should pay on average 86.4x because the setup is exactly like a bought bonus.
Technically it would be possible to track this if you noted specifically every 0 start win natural bonus though it's a lot of effort to prove something that isn't really all that important anyway.
The extra juice comes from the times where you're tumbling in wins beforehand or start with more spins.
From my data so far I have had 34 bonuses that were natural GOLD (+ 1 natural GOLD+5 which I have removed). Now I know that some of these will have paid a bit before but obviously tumbled GOLD will always pay something or it couldn't have tumbled while natural can (and more often is) straight in with 0 win.
So 34 natural bonuses have paid an average 89.7x.
Meanwhile I've had 67 tumbled in bonuses which includes +5. Those have paid 107.4x on average.
I know it's a small sample but the pattern that is starting to show from that is natural bonuses pay less because they are only ever 12 spins and more likely to have 0 or very small pre bonus wins. It's already quite close the the 86.4x buy figure and is probably explained away by pre bonus wins on the naturals that I haven't noted but will surely have happened.
I'll start to keep track of prebonus wins to get a better picture of this though.