I havent had a fire like you, but I have noticed that 300g is often just a little too much for the Gene. Last time I tried 300g, the roast was very uneven. So now I limit my roasts to 250g and have had no problems.
Last week I roasted 250g of Sidamo Koratie and I could actually hear the cracks. Compare with the roast I did tonight with Brazil Bourbon Bold- couldnt hear any cracks and stopped the roast when the desired colour was reached. And when smoke started to pour from the chaff collector!