I sat in on a Radar reflector test a few years ago - the findings were that it has a lot to do with the way the reflector is mounted - they DO help but not much - obviously the higher above the water the better - bit different on a yachts mast compared to a RIB.
One thing that was noticed in the test was that some boats without reflectors returned bigger returns than other boats with them - so many factors to take into account.
Remember a RIB is probably the worst thing you could have for being picked up on RADAR - low in the water - made of absorbant materials etc. Which is why drug runners love them!!!
The best bet for a RIB would be an active system - possibly something like the following
It re transmits the radar signal and also tells you when it has had a paint(signal)which is quite usefull.
Tests on radar reflectors can often be misleading - often the angle is very crucila - a reflector that gives a huge return on certain occasions can be all but invisible on others - bit like shining a laser pointer at someone.
This test looks ok but they all seem to favour yachts - a RIB is a bit different!!!