It doesn't really work that way. If you do a terminal MS, you will have to pay out of pocket. Generally, when you are admitted for a PhD, you start as a MS student, then once you pass your quals, you become a PhD student. The whole time, you get your tuition paid for.
As for working at Renaissance or DE Shaw, why bother? Tech is nearly as profitable for 90% of quants, and offers much better environment and long term career prospects. The dirty secret is that 90% of quants these days are just programmers. They aren't doing real research. Good luck having a 30-35 year career as a quant. Most likely, you'll get laid off after 10 years when your skills are outdated, and have a hard time finding new employment.
Firms like Rentech only report eye popping returns for their employee/founder owned fund, Medallion. The investor returns aren't nearly so good. Most likely what they are doing is taking all of the good trades/algorithms and putting them in the Medallion bucket, and all the shit left over goes in the investor fund buckets. Then when they recruit new capital or employees, they trot around Medallion and ignore all of the other funds that either sucked or got closed.
It's like asking how Steve Cohen can report such good results for so many years. Its not like all of the other firms, such as Citadel or 2Sigma can't hire the same PhDs as Rentech. Plus, look at LinkedIn or an alumni directory for who currently works at Rentech. Its really not nearly as impressive as the people who joined 15-20 years ago. The smartest people nowadays are working for Google and making nearly as much, sometimes a lot more.
Bernie Madoff, Steve Cohen,....Jim Simons? Notice a common theme? Investors might want to think twice about who they give their money to. That said, I think Jim Simons is smart enough to (a) not get caught (b) not do anything technically illegal. Instead, it will just be investors holding the bag. Its not alpha if you randomly find 1000 coins, and put all of the biased ones in the Medallion bucket.