Re: No Choice but Windows 8?
As others said, if you want you can still get Dell machines with Win7.
However: my primary machine is an HP laptop. Beefy, though (quad i7, 20GB RAM, large SSD). OS : Win8 (clean install). On this I have had running, simultaneously, my default applications, and a virtual test environment that combined 2 Windows 2012 servers (1 as DC combined with SQL 2012, the other as backup DC and Exchange 2013) and 9 Windows clients (3 XP, 5 Win7, 1 Win8). I have even had 2 additional XP vm's running on a usb3 disk. I haven't had any performances issues whatsoever, all of the vm's are default installs with indexing enabled. Granted, it's not terrible fast, but the client vm's don't run much slower than your average 3 year old desktop.
I'm not a developer, and I haven't really done extensive performance testing (so Exchange and SQL weren't under heavy load), but I am curious to know why you think the live tiles would "steal" that many cycles ? AFAIK, that would only happen when you have "live" tiles (which I don't), and are in the TIFKAM interface (which is practically never in my case). All the auto-update stuff is disabled too (including Oracle/Adobe and other third party stuff), but those things will steal cycles on any Windows version.
So please enlighten me : is your problem with Dell, with Windows, or what ? If you want to know at all times what your machines are doing, skip Windows (and most Linux distro's too nowadays, every recent OS does stuff in the background with a default install).