I used this calculator on my Dune Buggy build and it was pretty much spot on: http://www.compgoparts.com/TechnicalRes ... ulator.asp
Once you enter the sprocket sizes, motor RPM, and tire size you can play around with the sprocket ratios to increase speed or torque. The easiest one to change is the motor sprocket, of course.
Is this a 24V Currie? I see a couple online rated at 2,600 RPM at 24V. If that is the one, and you are using the stock gearing at 36V, you are looking at 3,900 RPM. If stock speed is 8 MPH, you should see 12 MPH. That is what we saw on ours when we went from 24V to 36V on the stock motor.
The stock controller is a 20A controller, so the 28A controller will give you some more torque. Not neck snapping torque, but should be noticeable. Running a 50A controller would definitely be an upgrade you feel, without smoking the motor.