I have the 75/35 plan with a MI424WR Rev l (80211.n) router. On any PC or a MacBook Pro I see ~84 mb/s down and ~38 mb/s up, via Ethernet or wifi.
However, with an iPhone 5, iPhone 4S, or iPad 2nd gen I only see ~40 mb/s down and 34 mb/s up. My take is that the iOS devices don't have the combo of hardware and software to process any faster speeds.