With a D300, the only way where 14 bits could make a difference is at base ISO, and even that is questionable.
There is a relation between useful bits and noise level. According to DxOMark, the maximum DR for the D300 is 11.72 EV (screen) at ISO 200, which means 12 bits should suffice.
Going above ISO 200 will make completely redundant the extra 2 bits from the 14 bits file
At ISO 6400, the DR is about 6.7 EV (according to DxO Mark), so even 12 bits is an overkill
For those kind of exposures you should use "Long Exposure Noise Reduction"
Also, I would try to use the same exposure time and f/stop and setting a lower ISO, like 1600, and then adjust in post processing
If you want a more detailed study, this study
analyzes the issue of bit depth, noise and dynamic range, including the specific case of the Nikon D300