I've been doing a $10,000 annual in-plan Roth conversion for each of the last two years and should do another one here shortly. That $20,000 I've converted to Roth ($10k January 2016, another $10k January 2017) have now grown to almost $27,000 with zero additional contributions (at least to that particular part of my 401k which gets categorized separately for my normal Roth contributions). I end up paying an additional $1,500 in taxes to make the conversion but the growth over the year that's now completely tax free more than makes up for that extra tax. This year it will only be another $1,200 in taxes - woohoo tax cuts.