OK, Trip, now I don't understand what you're getting at?! This is a TURN-BASED-STRATEGY game. How can it come down to the person who can 'click the fastest'?? Unless you are playing that pseudo-RTS system from Play the World, of course!! The best bet, IMHO, is to simply keep your units UNDER the stack limit as much as possible (as I already do in Civ3). Also, as the limits wouldn't be HARD, a player with an excellent stack (i.e., with a good mixture of bombardment, offensive and defensive units etc) is going to prevail over the player with the weak stack containing mostly one unit type-EVEN if the former is slightly overstacked for the tile he/she is in!
For example, LETS just say you have a stack of 8 units, in a tile which can support an optimal number of 6. This means that the units in the stack are all fighting at 10% normal efficiency. Now, lets say that this stack is made up of 3 offensive units (AS=10/DS=4 in civ3 terms) 3 Defensive units (DS=8/AS=5 in Civ3 terms) and 2 Bombardment units with a Bombardment strength of 8) This gives the stack (using your system, Trip) an Attack strength of, say, 42; a Defense Strength of, say, 39; and a bombardment strength of 16 (unless these last units remain seperate?!) Anyway, even with the 10% penalty, this stack is still operating at AS=38 and DS 35 (and, perhaps, Bombardment Strength=14) Now, lets say this stack comes across a stack of only 5 units-all of which are Offensive units (AS=10, DS=4)-now he has a AS=50, but only a DS of 20, which means that he will probably get successfuly hit TWICE for every hit he manages to dish out. Of course, if MY mass combat system were used, then three of the units from the latters' stack will be fighting two opponents, meaning that any combat penalty suffered by the first player, for overstacking, will be overcome by the penalty suffered by those units of the second player that are fighting 2 to 1 (not to mention the new hole that the two bombardment units are going to rip for player 2 EACH TURN

!). Hope that makes sense!! My point, ultimately, is that even with stack limits, it will most often be about the composition of the stack, rather than how much is in it. Its only where two equally matched stacks come together that overstacking will have the greatest impact overall!
EDIT: Of course, ANOTHER way to encourage stack limits-without forcing players into it-is if you have a 'visibility' stat. In this case, units would ADD their visibility stat together, this making it VERY hard for a 'stack of death' to remain hidden at medium ranges. A small stack, OTOH, would be much less visible to the enemy, thus making the chance of an ambush all the more likely-granting the smaller opponent a chance to 'even the odds' as it were prior to the start of combat in earnest!
Yours,
Aussie_Lurker.