If you are buying a retina model sadly no. If you are getting the last in the non-retina model we encountered issues in many as the optical drive SATA port runs slower than the HD SATA port (depending on the MHz of your system). There are other issues related to the anti-crash services that also make dual HD's or HD with SSD setups iffy on laptops. We gave up doing it as we had more problems than it was worth.
Frankly, if you really want to improve the boot time I would go with swapping out the HD for a SSHD unit Vs a dual drive setup. Seagate makes a good one.
You'er getting warm here ;-} It's the issue of the SATA spec of the given I/O port and the given device you are using on it.
Think of it this way you won't drive a Porsche race car on a dirt road nor would you drive a tank down the race track. Each has been designed to work within a given environment.
So in this case the current HD is in its ideal location - The current HD SATA port (Porsche & racetrack). Moving it over would be not ideal in some models as the optical drive SATA port is slower (Porsche & dirt road). And you still have your SSD which maybe slower than what the HD SATA port can use (tank on a race track).