Another thing to consider when using a chimney starter is how much charcoal goes in the chimney versus how much goes in the basket. According to their YouTube video, you only put a single layer of unlit coals in the basket and the remainder in the chimney starter. By my estimation, that's about 2/3 of the charcoal being prelit in the chimney, but this contrary to the advice the Glanvilles will give you. You really only need about 1/4 to 1/3 of the total charcoal starting out in the chimney while all the rest waits in the basket. Basically, you only need a single layer of lit coals covering the top of the unlit coals in the basket, the opposite of what is conveyed in the video. Once I started doing this, I began getting much better results with sustained heat over a longer period of time.
I too began using a charcoal other than Kingsford Original, but since I adjusted the charcoal lighting process at the same time I switched to Kingsford, it's hard to say how much of a difference the different brand of charcoal actually made. At some point, I'll try using my old charcoal brand again to see how much difference it really makes.