I don't think the premium 8's are available with a coaster brake, either.
The Nexus 7 is a fine hub, and according to an article at
http://hubstripping.wordpress.com/internal-gear-hub-review/, the gear ratios on the 7 are "stepped" more evenly then on the 8. On the other hand, I think the 8 may have less pedal resistance, than the 7, especially the premium version. Three of my bikes have Nexus 7's, and I dig them, but I have yet to try an 8, to compare.
Here's a few quotes from the article:
"The Shimano Nexus INTER-7 hub has the smallest step size between 13,8% and 17,3,%. Also the size is nearly the same. A absolutly fantastic gear step chart. This is the way a internal gear hub has to be designed."
"Shimano tells us they improved the Nexus INTER-7 hub to the Nexus INTER-8 / ALFINE. Sorry! That´s not the case according gear steps. Have look on the chart. The chart jumps down from 22,2% to 13,8% and once again up and down. This is no well balanced hub. If you want to buy a internal gear hub from Shimano take the well balanced Nexus INTER-7."
"Therefore we´ll have a lock on the second important characteristic for internal geared hubs. The size of steps between the gears.
In details there are two step charactersitics:
1. The size of the step between two gears.
2. The size of the steps compared to each other.
The size of the steps between the gears defines the chance to find a perfect gear inside your choosen overall gear ratio. Smaller steps are better then larger steps. With smaller steps you can finer react on your cycling environment e.g. wind, a inclination, a trailer…
This will increase your cycling fun!
To increase your fun even more the size of the steps should be the same. Think about a old stair in a castle or old house. The steps are worn out. This means the step size changes from step tp step. This makes it very hard and confusing to step up. It´s a very inefficient walk. The same will happen with a bicycle. When the steps beween the gears change cycling gets frustrating. You´ll never find the IDEAL gear.
Conclusion: The hub should have small steps beteen the gear and the step size should be the same."