Right, I agree that a dose of 8 mg / kg BW is pretty high. For me, that is about 600 mg of caffeine in one shot. If I am not well rested and hydrated that day, I risk getting jittery and possible heart palpitations. That's two cans of Spike Shooter in one shot. Again, if I were to even consider this, I would only use the strategy once every 7-10 days (assuming a carb cycling approach to diet) when I have a PWO, carb-up meal, along with something like R-ALA to maximize muscle glycogen uptake.
I am sure there is a lower dosage of caffeine that is still beneficial, but I also think there must be a point where such tricks aren't necessary, i.e. when your insulin sensitivity is already high enough to where the side effect of such a high dose of caffeine would not warrant the marginal benefit on glycogen resynthesis.
I just thought it was interesting and I am sure there are other compounds that can yield similar effects.