If Windows 8 installs native drivers, they should be ok, but I wouldn't count on all drivers for all needed hardware to be installed by Windows 8 natively.  You may need to find drivers from Dell or from the device manufacturer.  Windows 7 drivers will "probably" work ok in the absence of authentic Windows 8 drivers.  Also, not all native drivers are adequate ... Windows may find and install a driver for a sound card (for example) yet the sound service won't be able to start without the correct third-party driver, or Windows might install a video driver, but some resolutions and bit levels aren't available without the correct driver.
I bought an XPS 15 last year. I'm currently running 64 Bit Win 7 Enterprise. I'm thinking about putting 64 bit Win 8 Enterprise RTM on the laptop. Are there, or has anybody, experienced an problems with the drivers? Everything I've read online so far, which has been fairly limited, has indicated that the stock drivers that Windows 8 installs should work fine, yet I'm not sure yet myself.  Any thoughts would be appreciated. Eric
64 Bit Windows 8 Enterprise RTM on XPS 15 w/2nd Gen Core i7