I'm not sure what the advantage would be. The SD9 will at least double the recycling time too. The Lumedyne says it is a 1500 mAh battery. The SD9 has two sets of four batteries in 2000 mAh clusters ( dependin on batteries used) for 4000 mAh.
What piqued your interest?
Scott Chapin Powder Springs, GA, USA Nikonians Team Member
I just saw an ad for it in Shutterbug and was curious. I' m still trying to figure out a way to get better performance from my SB/900s. But maybe it 's better to just switch to Quantums ( I think my wallet just twitched )
Have the Lumedyne X and love it performs much like a mimi cycler for speedlights. Very small and compact and plugs directly into the flash so no cords to get tangled in. Performs as advertised doubles the life of the onboard batteries and cuts the recycle time in half. Not as fast as the big belt cyclers on full flash dumps but with higher ISO and TTL use you may not notice any difference. The X also maintains the flash capacitors at full charge for more consistent results. Normally the capacitors loose some of their charge before topping off again from the onboard batteries.
Performs very different and better than the cheep Chinese extra batteries that only extend battery time but do nothing to speed up the recycle time of the flash. Save your money and get the Lumedyne X it's a much more useful and a real photographers tool.
Once you get used to using the X or a Cycler and your flash fires every time at the correct intensity it's hard to settle for the basic performance and missed shots that the onboard batteries alone provide.
I find that the Lumedyne X now lives in my camera bag as a light weight alternative to a cycler battery pack. It's great to have cycler performance at hand without the weight penalty of dragging it around if you don't need it or use it. Great for traveling less stuff, less weight!
I'm a professional wedding photographer and found the Lumedyne X to be a great and useful addition to my bag. Very handy when dealing with two bodies, no tangled cycler cords. Also love the USB charger that lets me recharge in the car between locations.
Lumedyne, is also a great company that really stands behind it's products that are made in the USA.
The X does slow the thermal shutdown a bit as it draws the energy from the X and not the batteries. However nothing can solve Nikons bad design and overheating problems on the SB 900. For the flash to be a usable professional tool the overheat protection must be disabled so the flash doesn't shutdown at the worst possible time like at a wedding for the first kiss. Trust me the flash will overheat or outright fail at the worst possible moment every time!
The best solution to the SB-900 overheat problem is to use an external battery and shoot at higher ISO's on TTL allowing the flash not to work as hard. Also having a second flash to swap with when one gets too hot will keep it away from the Melville repair center a lot longer. I've done many weddings with a flash cooling down in my suit pocket.
The SB-800 is actually a more reliable and robust flash than its newer cousin, but only when used with an external cycler or flash accelerator & battery like the X. The 800 has a different overheating problem with the onboard batteries. When repeatedly fired with just the onboard batteries they will overheat to the point of melting internal components and the batteries become to hot to touch. This problem is completely solved when using the Lumedyne X or cycler. With an X or cycler the SB-800 draws all of its power from the external battery source and becomes a very reliable flash. The 800 is also smaller and lighter, a bit less powerful but with the higher ISO capability of todays cameras not much of an issue.
Basically any small plastic camera mounted flash will have real limitations in both power and durability when pushed in commercial use.