The RR will work for straightline and acceleration tests. I've played with them a little bit, and they're definitely more forgiving than the older GTech units. However, it won't help you effectively measure the benefit of aero mods unless you have a very long straight line strip to work with (say, a full mile where you can go full throttle). And even then, it is an accelerometer system designed primarily for low speed use, so it won't be great for that kind of testing.
If you are planning on doing autox and open track days, you're probably better off with a GPS based logger. Do you, by any chance, have a Nokia S60 or Windows Mobile phone / PDA? If so, it might even be as simple as getting a high frequency (5hz or 10hz) GPS receiver and hooking it up to something like RaceChrono (http://www.racechrono.com/). GPS receivers run between $50 ~ $100, and the software is free. If not, you could try something like a MaxQData TraQr (http://maxqdata.com/MQGPS.htm), which is a self contained recording unit that downloads data to a Windows desktop, laptop, or PDA. Both are relatively cheap, are very consistent, and require no modification to your car.