Hi There..
I ran with GRP last year..
I've used all new grp tires like Cayman, Plus, Grip, and so on.
I think that tires are NOW at highest level..
It's absolutely interesting that you can choice internal mousse (orange=soft, black=hard) and compound as your desire.
Best choice for winter (temp below 15°C) is X compund, that gives great traction and low wear
When it's hot (25°C or more), it's better to have an A compund.
About internal mousse:
Orange it's REALLY GREAT on bumpy track!! You feel the car really linked to the track!!!AMAZING
Black on smooth track..black gives higher speed, but only if the track it's very smooth.
I've used Aka, and proline, but prefer (thinking also at price) NEW GRP TIRE LINE..
BTW i buy them on adrmodel store