First off, there is technically no such thing as "GRP rubber". All the foam used in R/C gas car racing comes from a single factory in Japan. Everyone uses the same foam. Some tires seem to work better than others because of the rim stiffness, or offset. Maybe the durometer readings are a little off or the tires are older and firmer. Perhaps Brand X cuts the donuts smaller than Brand Y. These are very slight differences.
I really don't feel obligated to discuss where exactly Kyosho gets their tires made. Suffice to say that GRP does not do it. That is all that is relevant in this conversation. Contrary to what you think, Dredd, this is not causing trouble...it is called enlightening the consumer.