I decided to take a deeper look at IBKR's tracking of cost-basis, as I was not 100% confident from past statements when doing taxes.
From the current 400 shares I have already been holding of Sonos, the platform shows I'm just under $39 price-per-share for my average-price-per-share.
I went back and looked over the records on how the shares were assigned. Originally this occurred when four of my short puts got assigned at a strike of $30, with $408.00 in proceeds and $2.70 commission. That equals a basis of $405.30 for the premium (this value shows up correctly so far in the statements).
Now, according to the documentation, IBKR claims that it correctly takes the premium from assignments and bakes them into the basis for the shares of stock, so the taxman won't be upset when audit-time comes.
I did some manual calculations here, and sadly my value came well over $30 a share for average share price at first. Well, that was my silliness, as I was adding the original premium value to the basis of the stock, instead of subtracting it!
After adjusting things properly, my cost basis for stock was $28.98675
Now, this is very close, but not exact to IBKR's reported value of $28.9867388
Comparison:
28.98675
28.9867388
For a moment, I thought IBKR may not have been taking the small trading fee into factor here, but that turned out not to be the case after digging deeper.
Regardless, I am very happy with this, as I am not trading billions/trillions of dollars worth of stock so it's not going to make a penny difference with this minor rounding/truncation discrepancy.
My faith in the accounting (so far) is back to restored. Now, maybe later on sometime I will try to do a close analysis on how IBKR is really handling wash/superficial-loss trades.