First of all, an ECU doesn't increase REV limit, it just changes how close to, or how far above, the red line you can go. If you wanna really improve your car potential, you gotta do actual engine upgrades. A good ECU always helps though, so try to find a specialty place that can install one and get it programmed just right. If you can't find/afford a good ECU or specialty place, run a really powerful electromagnet over the cars computer. That will eliminate the 180 km/h speed limit, and it will rev as high as you punch the gas, so be careful not to blow the engine. Also be careful about WHAT computer you put the electromagnet over- you might screw up the fuel injectors among other things, but like I said, go to a specialty place.
As for the automatic transmission, manuals are obviously faster if you know what you're doing, but modern automatics are sometimes under rated. If you get the horsepower too high, you might end up breaking your automatic. If you're gonna switch transmission, save up for a nice sports sequential one.