I have maxed out my TSP(401K) and catchup for the past 8 years. At first it was a percentage, and through 2003, my contributions were considerably less than $10K, even including catch-up, though I would have contributed many times more had it been permitted. I think it was in 2006 that the contribution limit went from a percentage up to $15K + $5K catchup. That represented a huge increase in my contributions. Despite my cost of living increase, my take-home pay was a lot less that year!
This year, there will be no increase allowed for my TSP contributions. My cost of living increase and step increase will still give me a hefty increase in take-home pay, even after subtracting out enough to max out my Roth IRA contribution and over-50 catchup there too, again.
I wish I had had the opportunity to contribute more! Even more, I wish that I had a humungous pension in store for me like in the old days (I have one coming, but it will be really, really tiny).