Someone back during the original MTW was talking about attack and defense calculations being on a scale of dimishing returns. I don't know for certain, so I was asking you, actually.

The fixed bonuses were definitely straight numbers, not percentages, in original MTW. Considering that stuff like mount_effect bonuses are still straight bonuses in RTW and M2TW, I don't think it has changed all that much.

On the subject of fire rates, I might suggest that the crossbows and guns having somewhat more decent firing rates than would otherwise be expected (an early firearm could take as much as 150 seconds to load; it's really the much later muskets that could fire 3-5 times a minute) was made both in the interests of play balance and as an abstraction of the many 'shortcuts' such slow-firing troops would employ to increase their effectiveness. For example, crossbowmen would often have several helpers to load spare crossbows while they fired off their shots, while musketeers fired in ranks. I think CA has better things to do with their time than create crossbow loaders and such just to satisfy historical accuracy. Also, I think people were getting tired of the really slooooow rate of fire in MTW, where arquebusiers were effectively useless and crossbows took so long to shoot off all their bolts that the 'softening up' phase of every battle could take 15 minutes or more. Likewise, people were unhappy with how fast archers, especially longbowmen, were shooting off all their arrows, such that they then had no ammo left to repel a surprise attack by fresh troops.