Some of you guys really need to take a look at the unit stats in the export_descr_unit.txt file to see what's going on with missile units before commenting:

Your average slinger unit has a paltry Primary Attack rating of 4 with a Range of 80. Balearic and Rhodian Slingers are so deadly because they possess a Primary Attack rating of 9 with a Range of 120, making them the most effective slinger units in the game and more effective than basic archer units.

Basic archer & horse archer units possess a Primary Attack value of 7 for their bows with a Range of 120 making them only mildy successful against well protected troops. Generally the more expensive the archer unit the better the Primary Attack rating of the missile and the better the range. High value archer & mounted archer units like Persian Cavalry, Cretan Archers, Pharaoah's Bowmen, British Light Chariots Forester Warband, etc. possess Primary Attack values for their missiles ranging anywhere from 10 to 15 and ranges up to 170! The last few units will undoubtedly take a heavy toll on most armored enemies long before they come within arm's reach. In terms of pure missile power the Forester Warband unit is THE best archer unit of the game, effectively making them the Longbow unit of RTW. With Weapon Upgrades and maxed out Experience Chevrons these guys are long range killing machines that can punch through anything, even taking down War & Armored Elephants with only a few volleys...

Most javelin units of the foot and mounted variety possess Primary Attack ratings of 6 and 7 respectively. The Primary Attack values for foot javelins ranges from 5 to 9 with the highest value applied to all Skirmisher Warbands and Illryian Mercenaries. The Primary Attack values for mounted units range from 6 to 12 with the highest number used by Eastern General Cavalry. All javelins have a Range of 50.

The average pilum wielding unit possesses a Primary Attack rating of 13. The Primary Attack values for pilae ranges from 11 to 18 with the highest value applied to Urban Cohorts. All pilae have a Range of 35.

Simply based on the numbers it seems that the average javelin unit is no deadlier than your average archer unit but perhaps the fact that it the weapon is classified as a 'spear' gives it a boost against armored units. Pilae are also classified as 'spear' weapons and obviously make a big impression on most targets.

Beyond this, everyone's favorite Head Hurlers have a Primary Attack value of 17 and possess a Range of 40. And although it doesn't look like it their missiles are also categorized as inflicting 'fire' damage which would explain where they get their morale sapping effect from.

I use Adonys' half speed version of his excellent Kill Rate mod and I really like the effect. Melee deaths were simply too fast at the default setting but I do not feel missile units are way overpowered with the Kill Rate mod in effect. I think the one thing that people are noticing more than anything else is the extraordinary performance of high end missile units which clearly illustrate their superiority in battle compared to their plain vanilla counterparts. Perhaps some minor tweaking is needed here and there but nothing drastic.