I've got the MAIB's assessment on various radar reflectors saved.
Here's the link for those interested.
This extract, from the summary, makes interesting reading.
"It is recommended that poorly performing radar reflectors are not fitted as it
is possible that the user could be lulled into a false sense of security believing
that their chances of detection has been enhanced."
I guess the moral is that if you have a radar reflector, carry on assuming that you are totally invisible to any other boat traffic.