I have just done exactly that on my 16gb windows tablet today, and can confirm it works great.
Once you have doublespaced the drive and it boots properly, you will have to delete the old
recovery partition via diskpart. You cant do it in disk manager.
Run diskpart, 'list disk', find your internal disk, should be 0, and then 'select disk 0', then do
'list part', and find your recovery partition. mine was 4. the enter 'select partition 4', then enter
'delete partition override'.
Your partition should be gone. Then you can use disk manager to extend C: into the free space.
On my 16gb tablet, even with all the latest updates applied, I still have 7.8GB free
edit: note this includes hibernation enabled (size=50%) and a 1.8gb system managed pagefile and
only using the basic compression. This is a brilliant product, I was going to install programs on
SDcard but I don't need to, plenty of space on C:!
important edit 2: I noticed that when you delete the recovery partition, or perhaps as
a result of running zipmagic, that most of the options under troubleshooting (in advanced startup),
aren't there anymore. i.e., command line, restore from image, etc. But when you boot the recovery
usb you made from windows (you DID do this didn't you) then the options come up.
presumably they will also come up if you boot a win 8.1 setup usb.