There is no benefit cap, just a progressive rate. At low lifetime contributions, you get 90% benefit compared to contribution. As you increase your contribution this moves up to marginal rates of 32 and 15%.

As long as it's still a diminishing return and includes investment income, I have no problem with that. The real issue IMO, is it's simply not diminishing enough relative to expected lifespans, and there is a huge benefit associated with being married.

Sure, but you may want to add additional bend points to the benefit calculation.

OTOH, the current high-end contribution impact may be low enough that you don't; certainly a debate can be had on that.

Can you explain how that would further the goal of social security?