Actual amount of work the engine can do (horsepower) remains unchanged. Torque will show no difference at dyno either because final drive ratio will change but will also be compensated for with through the dyno (reading engine speed and measuring horsepower, deriving torque). The gain people get in trap speeds are from one of two reasons: more ideal gear ratios for their power band and better launching capability. A higher gear ratio will narrow your gears. 7000 in 1st will now be ~35 instead of ~40, meaning that the engine will rev through the rpm band faster. This is good to get into boost quicker, but will be negative in the sense that the engine will not dwell as long in the power making range if that makes sense. Horsepower is work per second, so if there are more seconds at max horsepower, more work will be done. The other negatives of gearing to 4.10's will be increased wheel spin (torque multiplying) and perhaps more drivetrain shock if tires will hold torque. I am just writing this down very quickly, so I hope you see my point. Everything is a compromise. Some cars will be faster with 4.10's and others will be slower -- it depends on the power band. I can elaborate more if necessary
