Rate 0.0008 btc per post looks good for Sr. Members, but overall 0.016 per week not really..
My advice is to make max limit of posts a bit bigger.. for example 30 posts/week and minimum 15 posts/week - this will attract more Senior Members. If you don't want change rates, will be better open this campaign for Full Members.
It depends on the budget of the company. But still a number of people are interested to join in that rates especially those people who have their day job and only find a part time of the day to search and discuss about bitcoin on this forum.