Where did you get to with this Greg? I have the exact same issue and I'm getting really tired of reading about doing PRAM or SMC resets
Same issues: kernel_task ridiculous cpu usage (500% sometimes); full throttle fan; unusable and slow computer; multiple disc formats and OSX upgrades and downgrades; doubled up the ram; disconnected good battery; heat sensors not excessive and nothing feeling hot. I've been hesitant to spend any more money on upgrades like you have without first exhausting any and all other possibilities. I've also been debating whether to buy a new computer altogether but other than SSDs the new MacBooks aren't that huge of a performance upgrade to this early 2011 machine. If I can figure this out I think a new SSD will be my reward.
I now use Macs Fan Control on manual setting. This morning I performed the delete .plist hack
Deepanshu posted about on January 4. Since I have El Capitan I had to go through a couple extra steps to unlock the protection like it says.
I did this earlier today and the computer has not lagged once all day. This is a first in months. The fan still goes full throttle but now at least I have full functional capabilities of the computer and kernel_task is staying at around 4 to 5%. That's a relief.
(That was strange: I just exited Macs Fan Control; my fan went full throttle but the computer is able to operate with normal functions. But when loading Macs Fan Control back up there are a couple sensors missing that were there just a few minutes ago: CPU Diode is no longer there. I'm writing this and troubleshooting as I go. I'll do a reboot and see what the sensors say after that. )
It looks like I need to use Macs Fan Control on manual Constant Value or on Sensor-based Value to keep fans from full throttle but I don't mind that at all. I'll keep digging.