I finally bit the bullet and upgraded my SP3 to W10 (still not liking W10 very much, but might as well start getting used to it).
I did get W10 to install without any heat, fan, battery issues. It took a couple of days of experimenting, but I think I got it. I have experienced the power, heat, poor battery life in previous installs/upgrades on my SP3.
First, download the current build from Microsoft via the Media Creation Tool (
Windows 10) and create and ISO or USB (you're going to do a clean install). Just make sure that you check the "Used recommended options for this PC". I didn't think this one checkbox would make a difference, but it did.
Once you have your media, BACKUP your data off to an external device (actually two separate devices would be a good idea, just to be safe).
Now do a fresh install of W10, and the one thing I found that has quieted down the system, and brought the battery life back to normal... Turn off, then back on Bitlocker on your C: drive.
After the OS is installed and you've logged in, turn off any network connection (you don't want anything to be downloaded in the background for awhile).
Open File explorer, right click your C: drive, and select "Manage Bitlocker". Then select "Turn off Bitlocker", and wait for it to complete. When it's done, reboot.
Now, you're going to turn back on Bitlocker. But this time select, when prompted for "How much of your drive to encrypt", select "Encrypt the ENTIRE drive (Slower but best for PC's and drives already in use)". Now you have to wait, and wait, and wait. I would recommend connecting the charger, and changing your power options to Never sleep when plugged in, and walk away from the computer, till it's done encrypting the drive.
Again, when it's all done, reboot one last time.
Do this, in this order, before doing any updates (OS or Apps) and restoring your data. I tried toggleing bitlocker after I had done updates and restored my data, but I still ran into overheating, slowness, poor battery life but it didn't work. It was only doing it the first thing, after a fresh install, that has made my SP3 (i7) run properly.