About the worst thing the current set up could be accused of is a gear range that isn't to the rider's liking. A different cassette and chain would cure that. (But this has been mentioned before). Not yet mentioned are the wheels and tires. Here the rider will feel a MUCH greater amount of rolling performance change. Although the loss of some "robustness" is a danger. Narrower/lighter rims, fewer spokes (be careful here..) and tires with lighter side walls and/or a narrower profile all help the responsiveness feel quicken up.

Getting more gears (from 8 to 9 or 10 cassette cogs) won't effect the measurable efficiency or the nimbleness of the bike. At the most more cogs would offer a gear ratio selection with slightly closer steps.

The biggest method of changing the basic nature of the efficiency of the bike is to change the rider/bike relationship. Flat bars reduce the ability for the rider to apply their strength, whatever level it is, and cause more wind drag with a more upright body. But whether the rider is up to the task of loading their upper body more is yet to be known. Andy.