Are there any consequences to under-estimating MAGI in order to reap the benefits of the lower cost sharing amounts? Assuming you do make estimated tax payments to "make up for" the eventual reconciliation of the premiums...
Not really. It's up to the exchange to determine whether your estimate is credible or not, and if they think it is then you're all set. If you habitually underestimate your income it's possible they could eventually put you under some more scrutiny and ask you to prove that you're really not going to go way over again this year before they sign off on it. Beyond that I'm not aware of any likely consequence to underestimating.
I think seattlecyclone is correct (as usual). There is a clause about "intentional or reckless disregard for the facts" in the instructions for Form 8962 (
https://www.irs.gov/pub/irs-dft/i8962--dft.pdf page 8 top of right column), but that appears to only apply if one overestimates AGI to qualify for ACA subsides in situations where one's AGI would otherwise be to low.
In my state, the ACA marketplace has no mechanism to see my federal tax return from the prior year, so there is no feedback loop to catch this kind of situation. The lack of CSR reconciliation on Silver plans with estimated AGI under 250% (or 200% or 150%) is just a bit of a not-very-well-known loophole.
Some people might object to this underestimation strategy on ethical grounds.