+300fpm at 10k feet is probably average for a relatively low power non turbocharged spamcan type. I think my TB20 would manage maybe +600fpm but it has 250HP to start with. A normal ~250HP turbo tourer would do +1000fpm (my sea level climb rate) all the way to 15k-20k.
The problem is that if your optimal (test pilot determined) climb rate is just +300fpm, the plane will be not far from stall up there, and controllability is not all that great. It's a bit like practicing slow flight in a C150 at 50kt
You don't really want too much turbulence...