From Discovery.com
Time flies on beta Pictoris b, a behemoth gas planet orbiting a young neighbor star about 63 light-years from Earth.
A day there lasts just eight hours, making beta Pic b a faster spinner than even Jupiter, which rotates in 10 hours. Pic b is the first planet beyond the solar system to have its rotational rate clocked, scientists said in an article published in this week’s Nature.
Technically, what is interesting is that it was so easy to do these observations,
lead researcher Ignas Snellen, with Leiden University, Netherlands, wrote in an email to Discovery News.
The discovery bolsters theories that the mass of a planet directly correlates to its spin, with bigger planets rotating faster than their smaller siblings. Until now, however, scientists had observational evidence only from planets in our solar system.
Scientists suspect Pic b, which is still warm and young, is just ramping up. Over the next hundreds of millions of years, it is expected to cool down and shrink to about the size of Jupiter.
If its angular momentum stays the same, the planet should spin up to about 25 miles (40 km) per second, Snellen notes in the Nature paper.
“This eventual spin velocity agrees remarkably well with the solar system trend,” University of Arizona astronomer Travis Barman noted in a commentary on the research, also published in Nature.
“The number of directly imaged giant planets will soon increase substantially, and many of these planets will be excellent targets for spin measurements in the near future,” Barman noted.
Read the full story
Tallbloke's Talkshop 
‘The discovery bolsters theories that the mass of a planet directly correlates to its spin’
Doesn’t work for the inner planets of the solar system.
How good/bad for the outers?
The rotation rates are in order of mass, and your PRP paper quotes the phi ratios of the rotation rates. But Jupiter has more than 3 times the mass of Saturn and only a few per cent faster rotation.
The inner planets are much closer together and tidal locking is a factor.