There isn't much worth in downloading a torrent copy of Ultimate since the main differences between that and Home Premium are increased network support and a few other items like creating a shadow copy of the existing installation. That's why the price tag was seen for about $140 originally.
As far as dual booting with two separate editions it will depend on how you set that up while each can see the other added into it's BCD file which acts as the boot information file there. A free tool called EasyBCD will take care of seeing one added into the other if you install the second as a stand alone on a second drive while the first is unplugged which would be seen in a desktop.
But you can still use that for renaming the entries and setting which would be the default OS with a gui not the command line BCD editor seen. That's found at Download EasyBCD 1.7.2 - NeoSmart Technologies
When switching back to ahci you will need to have all the necessary drivers onhand otherwise you may end up seeing a big BSOD real fast. You'll want the latest sata drivers ready to go. For the performance gain you have to pay a little there.