There is a gray zone for those in the mid 50s earning over the IRS cap or those who don't expect to exceed the cap until then (ie hired at 40+)

I've spent too much time thinking about this, when it won't move the needle either way. I'm tempted to opt in in search of a tax arbitrage "win" and ALPA dues wash with in service withdrawals. The time horizon to 59 1/2 is just a little further than I'd like. Once we have more answers on the in service withdrawal mechanics, I'll make a decision.