Late to the party on this one, but here's one thing I would caution about the L Funds, and especially the longer term funds, like 2040 & 2050. If you look at the fund sheets, they have a rather high standard deviation for the expected returns -- for the 2050, a roughly 8% expected return with a roughly 17% standard deviation. See:
https://www.tsp.gov/PDF/formspubs/LFunds.pdf27% of the L2050 is currently allocated to the I Fund, and it stays above 20% for a long time (use the pinwheel deal on the TSP website to cycle through the future allocations). Since 8/31/1990, the I Fund has returned a mere 5.9% annualized return, with a whopping 18.1% standard deviation. Compare that to the C Fund over the same period -- 10.2% return, 18.2% std dev; and the S Fund -- 11.3% return, 20.1% std dev. In other words, for a 5.9% return an I Fund investor has taken on the same amount of risk as a C Fund investor, who earned 10.3% For only slightly more risk, the S Fund return is nearly double that of the I Fund. See here:
http://www.tspfolio.com/tspfundsNo way I'd ever consider having 20+% I Fund in my portfolio. It's just not worth it. Also, the I Fund is a pretty antiquated fund insofar as what markets it tracks. It skews heavily in favor of Europe and Japan, and barely touches emerging markets like China and India. I've been as high as 10% I Fund, but am now 5% and considering dumping the I Fund altogether (many of the S&P 500 companies are multi-national corporations, so there's plenty of exposure to international markets in an S&P 500 or TSM index fund).