I am wondering why there is no middle ground in term of amount of domains?
The jump from 5 to 100 domains seen very high, i want to use more than 5 but lot less 100. I would like to start with about 25 and see how that goes, if it works i could then move up to greater amount.
Have you thought about lowering price of professional version and having domain license pack addition of say 25 more domains?
I suspect you would more sales this way!