Since amending would benefit me and cost the IRS/Illinois (please correct me if this is wrong), is it reasonable to assume that I am not required to file an amended return?
I think so. I have never found an authoritative statement, but, unsurprisingly, pretty much the whole tax code, related regulations, and IRS (and state tax department) policies are focussed on preventing people from paying less than they owe. I have never found any penalty for paying more than you owe beyond the fact that you paid more than you owed.
I'm not familiar with IL and don't know if they even give QBI (directly, or by using Federal items that include it), so you may or may not actually have a change there.
Also, would the benefit to me be $108 (Amount) * 20% (QBI) * my marginal tax rate?
Maybe, nearly.
It defnitely reduces your taxable income by $108 x 20% = $21.6 ~ $22.
If your ordinary-rate taxable income is more than $100k then your tax is computed by formula, and if you use the whole-dollar option (which AFAICT all sane people do) it is then rounded to the nearest dollar, so your saving is marginal rate x $22 plus or minus up to almost $1 depending on how the rounding lands.
Otherwise, your tax is 'computed' using the published tax tables. These tables use nominally the same formula, but quantized into $50 brackets. Reducing your taxable income by $22 has a 28/50 chance of remaining in the same bracket and changing your tax not at all, and a 22/50 chance of moving you down one bracket and reducing your tax by your marginal rate x $50, again plus or minus rounding.
In simple cases your ordinary-rate taxable income is just your taxable income on line 11b, but if you have preferred-rate income -- and someone receiving 1099-DIV likely does -- only line 11b minus the preferred-rate income is ordinary-rate taxable. The tax on the preferred-rate portion is always computed by formula (then rounded), either on the Qualified Dividends and Capital Gains Tax Worksheet in the 1040 instructions (affectionately known as QDCGTW) or the Schedule D Tax Worksheet in the Schedule D instructions. As the former name almost implies, your preferred-rate income is normally your qualified dividends and net long-term capital gains -- but you have the option on form 4952 to give up the preferred-rate on some of this income in exchange for being able to deduct (more of) your investment interest expense. {rhetorical} Aren't you glad Congress gives you such fun and exciting choices? {/}
