When reading this, keep in mind that Nikon recommends (p. 319 of User's Manual) Speed Class 6 or better.
It appears that much, very much depends on the cards's bus speed. As it stands now, there would be little benefit in speeding up the camera's bus interface beyond that of the minimum card supported, which would be Class 6 at 12.5MB/s. A firmware upgrade wouldn't get that bus interface any higher, not unless the hardware that implement that bus already supports the higher rate, and that's doubtful.
SD memory card manufacturers use different types of flash memory to create an SD memory card, so actual transfer speed can vary.
The SD Association defines Speed Class standards that pertain to bus interface speeds, not maximum data transfer, or burst, speeds.
The Speed Classes defined by the SD Association are Classes 2, 4, 6 and 10, with separate UHS Speed Classes I and II designed for UHS devices only. UHS (Ultra High Speed), the fastest performance category available today, defines bus interface speeds up to 312 Megabytes-per-second (MB/s) for greater device performance. It is available on SDXC and SDHC memory cards and devices.
Speed Classes available in SDHC cards: - Speed Class 10 uses a High Speed Bus Interface (I/F) intended for Full HD video recording and HD still consecutive recording - Speed Classes 4, and 6 use a Normal Bus Interface definition intended for HD through full HD video recording - Speed Class 2 also uses this same Normal Bus Interface definition, but intended for SD video recording - UHS Speed Class I uses a UHS-I Bus I/F intended for the full higher potential of recording real-time broadcasts and capturing large-size HD videos - UHS Speed Class II uses a UHS-II Bus I/F that is about a factor of three (3X) faster than UHS-I
SD memory card manufacturers created “maximum” or “burst” speed designations to support consecutive picture-taking in burst shooting modes on your camera. Maximum speeds indicate read/write speeds in megabytes per second, and is not defined by SD standards. Manufacturers indicate maximum speeds as well as capacity, Speed Classes, and Bus Speeds on the SD memory card and/or the SD memory card packaging.
It was in a press release at LAS VEGAS (CES Booth #14200), January 5, 2011 that Sony Electronics announced its SDHC Class 10 memory cards, which have a 22 MBS transfer rate, the fastest SDHC cards available from Sony.
I can't even find the max transfer rate (burst speed) on Centon's website, nor on its packaging for their Centon 8GB Speed Class 10 SDHC cards (Manufacturer Part # RC8GBSDHC10).