I really think it might be hard to justify with 10 or even 15 minutes turns. Even in 10 minutes, good infantry would fire minimum 20 volleys from smoothbores, but more likely 30. Thus a bonus for the first, and only the first, shot within even a 10-minute turn could not justifyably be higher than 1/20 = 5% of the standard effectiveness. You'd hardly notice.
Edit: Just to elaborate, that +5% would mean that you consider that first volley a full 100% more effective than the following ones. If it's only 50% as effective, the increase would only be 2.5% over the course of a 10 minute turn.
For a 15 minute turn, and seeing how 3 volleys per minute are more likely realistic for good infantry, even a 100% bonus for the first volley would result only in a 1/45 increase of the standard effectiveness, or 2.22%. If it's only 50%, obviously, the figure would go down to 1.11%. Whatever the calculation, the effect would be so negligible as to hardly warrant the trouble of introducing it into the game.
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Maj. Gen. D.S. "Green Horse" Walter
~ 2nd Dragoons (Royal Scots Greys) ~
2nd (Union) Brigade, Cavalry Division, Anglo-Allied II Corps
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~ 3rd (Prince of Wales's) Dragoon Guards ~
[url="http://home.scarlet.be/~tsh40803/AAA/BritGuardHorse.html"]

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