I've yet to see any real specs published to be able to directly compare ratio differences...But if they only changed certain gears then my hypothesis is probably correct.
The gear ratios are available on Mazda's website... :huh:
Here are the ratios ('07-09 / '10)
1st: 3.538 / 3.21
2nd: 2.238 / 1.91
3rd: 1.535 / 1.37
4th: 1.171 / 1.03
5th: 1.085 / .95
6th: 0.853 / 0.79
Final (1st-4th): 3.941 / 4.19
Final (5th-6th): 3.350 / 3.46
Don't forget, they also changed the tire size from 215/45R18 to 225/40R18.
In the end, the overall percentage of change (combining gear ratio, final drive and tire size) is:
1st: +1.72%
2nd: +7.92%
3rd: +3.20%
4th: +4.71%
5th: +8.29%
6th: +2.37%
The gear change means the '10 can hit 60MPH in 2nd gear, but not until 6500 RPM. This is generally well after the throttle has been closed by the ECU. However, it may still help 0-60 times. Also, the '10 can hit 100MPH in 4th gear at 5,800 RPM. Assuming the '10 has similar acceleration in the 1/4, it may be that the '10 can hold 4th for the 1/4 mile giving it a slight advantage.
However, this is pure speculation as I don't know how much of an affect the increased gear ratio means -- taller gear means less acceleration. However, as we all know, 1st and 2nd gear already have traction issues, so the taller gear may help reduce that and actually put power down and help. Again, more speculation.
If you're curious about '10 gears (including final drive) in a '07-09 with the stock '07-09 tires:
1st: +3.67%
2nd: +10.21%
3rd: +5.39%
4th: +6.93%
5th: +10.58%
6th: +4.54%
If I could afford it, and it were possible, I would take 1st and 2nd gear and 6th gear from the '10 and keep everything else. I think this would be a good mix, but of course, this is based 'on paper engineering' which may or may not play out in the real world.
Notice: I am not perfect... I may well have messed up the math. Don't jump down my throat. Also, I have gear ratios going to three decimal places for the '07'09 and only two decimal places for the '10. This may skew the numbers slightly. Again, don't jump down my throat.