If you want the factory overclock, you should purchase the Hydro directly. This guarantees the card will run at those clocks, and if it doesn't, you can get a warranty without issues.

The only issue with buying the reference card is loading any bios, other than the one directly from the factory risks warranty issues and the card may not be stable at the Hydro specific clocks. If that happens, or for some odd reason the card dies because of the bios switch, then you may have issues. It's easier to avoid them if you are going to spend the money on the Hydro kit later to just save a little longer to get them right away.

If you are comfortable flashing bios', you could get the reference and later buy the Hydro kit, and then flash a bios to make it run a little faster than stock. Once you do this, you have to make sure you save all of the original hardware so that you can return the card to 100% stock if you have to RMA the card. Losing the screws is a $10 charge +$6 shipping. Losing them for two cards would cost $26 just to start, and without them your card would be refused a warranty claim. So make sure you save them in a secure location. Generally, installing them in the cooler after it is removed and placing the cooler in the factory box is a good practice.

Basically, you have options either way. It mostly depends on how comfortable you are with the hardware changes and firmware changes.