Making such bows Exotic weapons would help, thereby requiring wannaba-machine-gun-snipers(I mean - archers!) to waste a feat beforehand. I'd suggest leaving normal longbows (1d8) and shortbows (1d6) as martial weapons though at least.
In the For More Than Glory campaign I'm presently substitute-DMing (Emiricol is busy lately), set in The 13 Kingdoms, I've seen Valdir (our party's resident fighter/ranger/deepwood sniper - though deepwood snipers are renamed in T13K to fit the elven group that trains them) nearly kill a juvenile green dragon all by himself in about 5-8 rounds, at which point the dragon fled up a tree and then flew off above the forest canopy, dipping back down again once it gained a bit of distance (trees below the canopy granting significant cover at that distance), as Valdir climbed a tree to try finishing it off. His range and attack modifiers are insane, at around 10th-level he can hit AC 20 or so even on a natural roll of 3, from a distance of like 200 or 300 feet; he can even manage fair accuracy from 1,000 feet. And he regularly deals around 10-12 damage on average per arrow, 2-3 times per round, and scores critical hits usually 1-3 times per battle (in the moderate-or-longer battle anyway). He uses alchemist's arrows often to bolster his damage output, since his Strength is around 15 or 16, while his bow only has a +1 enhancement so far. It's tough trying to come up with anything that will reasonably challenge the party when Valdir alone can easily deal out 100+ damage in 3-5 rounds, concealment or cover be damned. Creatures with some kind of piercing resistance, or miss chances from stuff other than concealment (i.e. Improved Invisibility, Displacement, Blink, etc.) could perhaps help.
