I have found Renjin quite useful in the past, and I love the motivation behind the project. I know that the guys at Bedatadriven hope to improve upon its performance, however it does not always (or often, depending on how you use R) outperform GNU R. Some great changes have been made lately (http://www.renjin.org/blog/2015-06-28-renjin-at-rsummit-2015...), so I hope to see Renjin's performance progress beyond GNU R across the board. I actually contributed Renjin's current PRNG – a Java translation of GNU R's – which was my first experience getting under R's hood.

The Purdue project you linked looks quite interesting. Unfortunately, development appears to have stagnated: https://github.com/allr/purdue-fastr

[edit] Another important aspect that Renjin contributes is the packages ecosystem: http://packages.renjin.org/