Take a look at this:
Use 2803 Cooling hours for Houston.
For KWH cost, make sure you divide the KWH on your electric bill with the final $. This gives you the true cost per KWH after taxes and fees, fees and fees, because that's what you really care about - Not the KWH cost on your bill.
Your house is not exactly small. Your electric rate sounds much higher than average. Yes, your systems are oversized. I think your bill sounds about right for the info you provided (It doesn't sound like the heat strips are on because the added loss would be double). A single story 4216 sq ft house has a lot of surface area to gain heat with. Replacing two 2 year old systems with smaller systems would probably have a 20+ year return on investment.
I would look at some fancy ductwork to twin the systems to have one system as the primary (with a higher cooling setpoint), and the 2nd system (with a lower cooling setpoint) running on a separate thermostat with large hysteresis to come on when the main system can't maintain it's set point. That's just me. I don't think many HVAC companies would be able to properly re-design and alter your ductwork for this. This wouldn't necessarily yield the greatest comfort for when 5 tons wansn't enough, but the 2nd unit would probably only cycle on and off for 3 hours during the hot season. I'm just thinking out loud.