15 question/day is not a metric that many SEI betas are capable of acheiving. Most of them are too niche to attract that kind of traffic and attention in a reasonable beta period.
However, if you look at recent sites that have graduated (I'll be looking at RPG, Travel, Judaism and Mathematica) you can see that the bar isn't nearly that high.
RPG - this is the site I'm looking at that I was most involved in when it graduated. When it graduated it had just 3.4 q/day. The big deal was the excellent quality (and quantity) of answers that were provided and the fact that the traffic was consistently rising (a healthy 1667/day when we graduated). This is a pretty excellent comp for your site as it's a niche (but growing) hobby and survived a very extended beta.
Judaism - This site graduated about the same time RPG did and had a much better number of questions (6.6/day) but more depressed traffic (758/day). Not a great comp as it doesn't occupy the same sector, but also had an extended beta.
Mathematica - This site sailed through it's beta and is one of the fastest launches of late. But you can see why from the stats 12 q/day and 1307 visits/day. This is a poor comparison to your site as it doesn't occupy a niche and is a professional tool rather than a hobby site. But gives you an idea that even the best sites don't have to live up to the 15/day metric.
Travel - Another poor comp but included to show that massive traffic means a lot more than questions/day.
I encourage everyone to check out the launched sites tab of A51 to get a realistic picture of what you're shooting for. Especially pay attention to recent launches of things that had extended beta periods (RPG, Judaism, Skeptics, Sci-Fi). But also be sensitive to the fact that SEI evaluates each site individually and looks more at trends than the numbers that they show to make their evaluations.