How big is the largest flame you've ever rendered? What resolution/quality?
To stress test my tiling algorithm, I've been rendering various flames from the Electric Sheep archives at 16,000x16,000 at quality 5,000. I also did one at 12,000x12,000 quality 20,000. Since I've greatly improved large image performance since version 0.36 (mostly due to an improved memory allocation scheme, which allows me to use all available memory on the device, even for ATI cards (I hope - haven't tested this yet)), my render times ranged from 2-3 hours for each image running on a single Nvidia GTX570. Unfortunately, there's a bug in the OpenCL implementation which prevents me from rendering anything larger than ~16,000x16,000; it appears to be a 64 bit issue since this corresponds to 4GB of accumulation buffer. I guess someone on the driver team tried to store a pointer in an int.
I would upload the images for you guys to look at, but they're rather large, as in >400MB each
