I'm not sure I agree with you. You said that drives are a lot faster than they used to be, so it seems like you're saying that in the past it was faster to have Windows and the pagefile on separate disks. Whether or not that was what you were implying, it is true; if I took two old HDs and split up my OS and pagefile between them, it would be faster than having both on one old HD.
If disks have gotten faster, than sure a pagefile and OS on a single new HD would be faster than the pagefile and OS split up on two old disks. Disk speeds and caches have more than doubled and octupled, respe