Minie balls were slower to load that round balls in a smoothbore musket because you had to hold them the right way up and slide them in rather than allow them to roll down the barrel. But the point was that you loaded them into a rifle, and the alternative there was hammering a super-calibre round ball down the barrel, deforming it in the process. That took a long time.
What has me wondering is whether a merely flawed bullet would be dented enough to expand on firing. Minie bullets had almost hemispherical indentations.
The important thing really, is that the minie ball only makes sense in a
rifle.
It would thus only make a peripheral change to history since it would be restricted to the few rifle/jaeger type specialist units that already use rifles.
In order to use a rifle effectively, you need to be taught marksmanship and spend some time on a range firing off quantities rather expensive ammunition. Each year, your riflemen have to have some top-up range time to keep their skills up to date.
In this day when nitrates are a rare commodity, manufactured from human and animal excrement, gunpowder is a relatively scarce and hence expensive commodity. The discovery of the Chilean guano island deposits in the 19th century helped this, for those who controlled the trade routes of course. but up to the Napoleonic era, you did not expend gunpowder needlessly. e.g. some Royal Navy captains believed in gunnery practice with live ammunition and not just dumb show gun drill - this had to be done with privately purchased barrels of powder.
Jaegers operated in loose order as light troops - which required more initiative and training for these guys, and also a greater quantity of (expensive) officers and NCOs to supervise such a unit as compared to a line musketeer unit. Thus they were rare because they were expensive to train and maintain.
Riflemen were a useful thing to have, but generally did not prove a decisive arm - or the armies of the day would have scaled up the fraction of troops so equipped. Riflemen needed a clear line of sight to the target, and fired slowly (if more accurately shot by shot). Good for bothering an enemy battery that deployed too close to them perhaps, or for whittling down a musketeer unit which did not/temporarily could not do the sensible thing in opposition to them (close to short range and ripple off a few rapid volleys into the gunsmoke, then close with the bayonet).
Line Musketeers did not have to fire
any live ammo whatsoever in order to keep their skills up. All these need is to be thoroughly trained in the motions of loading, and holding the piece level to discharge it on command once that process is complete. They only had to be trained in close order drill in tight formations that required a reasonably small number of officers and NCOs to manoeuvre the massed units about. A musketeer battalion was basically a giant shotgun, really. In battlefields wreathed by gunpowder smoke, with visibility less than 100 yards often enough, the musketeer units made perfectly good sense since their ability to put out a hail of shot into a "beaten zone" at 3 rounds to the riflemen's one tended to be decisive on the battlefield of the day.
A more useful technology to improve your musketeers (and jaegers too) would be an earlier introduction of the
percussion cap. That reduces the motions to load your firelock by about half, does not require any priming of the pan with part of the charge etc, and once loaded it's relatively rainproof compared to a flintlock. The percussion cap lock also is more reliable, so you would produce less misfires per volley. You might gain one or one and a half shots per minute over opposing flintlock musketeers perhaps, which would help a lot when the point of musketeers is to put loads of lead downrange as fast as you can.
In fact I'd say that the use of the widespread use of the percussion cap in the ACW period was perhaps more important than rifling and the minie ball part of the "rifle musket" combination. After all - most conscripts in the ACW did no range training, just "by the motions" dry loading drills in camp, and only ever saw their first live rounds on the day of their first actual battle. Learning to aim the thing was thus done "on the job", as it were.
Add the American preferred load of "Buck and ball" to your smoothbores (3-4 buckshot as well as the ball) in the European armies and you probably have made your musketeers 40-50% more effective per unit of time. All that without any bothersome (and costly) individual marksmanship training or any need to buy in rifles that cost perhaps 4-5 times what the smoothbore musket would

!
http://en.wikipedia.org/wiki/Buck_and_ball - see esp. paragraph beginning "Claud E. Fuller"