You know, just when I think I understand you go and toss in another consideration.... LOL But I think I understand what you are saying... doing what you suggest is then a trade off of the increased subsidy. Using my example dropping my MAGI from $68K to $61K, which would result in $1,104 savings.
By maxing your Roth contribution you then forgive this $1,104 savings. Aren't you then just paying the tax now ($1,104) to move money to your Roth? Maybe in the long run it works out as you then let the investment grow on a tax free basis. Something I'll have to model. Thanks for making my head hurt a little bit more on a Friday night
I think of them as separate decisions. Does an HSA plan/contribution make sense? Separately, do I want to do Roth conversions, and how much? The "how much" may be influenced by the HSA contribution, so there is some tie in.
Let's take it as a given that you'll be doing the HSA contribution.
Many people do Roth conversions up to the ACA cliff, or up to the top of a certain tax bracket. If you're already doing a Roth conversion, you've decided that the tax paid now is offset by the tax free growth, and avoiding possibly higher tax rates in the future.
The income reduction you get with the HSA contribution just makes a little more room in the same space you were already using. So while you might see it as giving up the $1104 tax savings you were getting, another view is that you are paying $1104 to convert that money now rather than paying higher taxes later.
Here's an example where you wouldn't convert the same amount that you put in your HSA. Suppose that without the HSA, you'd be $3200 over the subsidy cliff. But when you do the HSA contribution, you are now $4000 away from the cliff. If you then decided to do a Roth conversion, you would only convert $4000, not the full $7200. Of course you would leave some safety buffer, but it shows that you would treat them as separate decisions.
I have a spreadsheet each year to figure out how to manage income for the ACA subsidy, and for estimated taxes. When I have an HSA plan, I subtract my contribution from the income number in January when I make the contribution. Throughout the year I add in income. Near the end of the year I figure out how much, if any, Roth conversions I can do. So while my HSA contribution and Roth conversion amount is related, they are separated by 11 months.