If you have fine enough GPS coords you don't need the accel data, since acceleration is variation of speed. However, GPS is not accurate enough to give anything else than a somewhat average speed, since errors prevent it from detecting sudden changes, like peak acceleration and stuff. A compound data log would be nice, but apps need to be rewritten (or running at the same time). Precise time stamps would be much more useful.

0-100 and 1/4 mile is a lot easier with a watch, since the sampling of the program is imprecise and to get speed you need to add acceleration every second, a process that compounds errors. I'm pretty sure you get a lot better with a watch.

What you CAN so is start recording, floor it to 100 and then suddenly take the foot off at 100, and then see timing, so you can clock yourself. However, I usually do this via camera (video) because frames are a lot more precise. used my N80 that did 15 frames per second and counted frames from the speedometer first nudge to 100 Kph (60 mph). However, you can time yourself to A speed (not necessarily 100) by adding acceleration and delta timestamp. It will be AROUND 100, and probably fairly constant through several runs, but unlikely to be exactly 100 (kph).

What the accel log will show your is a power curve as they like to call it (actually it's the torque), and thus profile engine and gearbox (and driver), see where it's less than efficient and either tune it or learn to redline race. The dyno is better because the measure "at the wheel" which is good but has nothing to do with what's in the owner's manual.

What's even more useful for is profile the route for a race, log attack angles and lateral acceleration limits, profile tires, stuff like that (just like pit crews, except they log speed from speedometer not GPS and have may other sensors (you don't need) like suspension excursion). The GPS is useful at high speeds and averages only at current sampling rate.

As for HP and stuff, that's marketing alone. Even the original app says "guess" your horsepower. Because of how one's HP is calculated, it has nothing to do with acceleration (hint: locomotive). Dynos make use of an adjustable hydraulic or electric brake to variable-load your engine - something you can't do alone.

Also, since it's passed through the transmission to wheels, torque is also hidden unless you know the gear ratio, final drive ratio and exact tire specs (including pressure).

What it's really good to have an accelerometer for is profile tires (which give me better traction), limits (skid, lateral and forward), and tuning. That's where the "dyno" thing comes in - if you drive really well you can see if a new sports filter gives you a torque increase at what RPM - something that would be lost with other measurements.

You can also compare runs of the same track and see what late or early braking brings, for example.

For me, it's going to be a race for peak G at the start line - I'm a bit of a forward G junkie. At 0.9 G stuff that isn't bolted practically fall to the back of the car. That kind of stuff you can't (and shouldn't need to) explain.

Oh, and, the car those figures came from is a 3-liter V6 and RWD on wet, cold asphalt. I can do better.