When I first got the router I spent a bit of time postioning the antenna for the best performance - alot of trial and error! What worked best for me was having the router close to the floor because it's upstairs, and having the 2 side antenna pointing down at slight angles and the middle one pointing up, even on the 5ghz network I have full signal around the house. but the speed does drop ever so slightly
The positioning of the antenna is important to gain optimum performance, and I found it made ALOT of difference to the speeds.
Oh and just to add, the iPhone 4 is not 5ghz compatible so I'm guessing that your mini was still on the 5ghz while your iPhone was on the 2.4ghz and thats why it was still faster than your mini. Put your mini on the 2.4 and I suspect it will be the same or faster than your iP4.