Generally, while a Rogue can get Sneak on every attack in a round, you can't count on that every round. You need a flat-footed opponent.
You can use concealment, but as soon as you attack you blow your cover, and need to hide again, and that's not a Free Action. One attack per round.
You can use Flank, but it's not always available, and some opponents will move out of it, forcing the Rogue to move to re-establish it. One attack per round.
Improved Feint can cause an opponent to be flat footed every round, unless they move away from you. But it uses your Move action, so... one attack per round.
In general it's comparable to Skirmish, which requires 10 feet of movement. It does more damage, but not as often.
The Skirmish limit can be bypassed with a Carpet of Flying: It needs a command word to set it in motion, but once in motion its movements are controlled by spoken directions. Speaking is an immediate action, so the carpet moves, and the Skirmisher still gets a full round attack. You can't use Mounted Combat to bypass, but the Carpet works.
Greater Manyshot aids both Rogue and Scout, but it's far easier for the Ranger/Scout combo to get it than for a Rogue. The Ranger/Scout also has a better BAB (Ranger gets full Fighter BAB, so the character is only one point below max, since Scouts start with a +0 at level 1.)
The character in question, with a Mighty bow +2, +1 magic and a die of energy damage, will do a D8 +3 +D6 energy +3 D6 Skirmish. He'll average damage in the low to mid 20s per hit. Since this all takes place at full BAB, that doubles the average to about 45 per round.
The Rogue using the same feats will still do more damage, but even less often. Cover and surprise work, but flank is off the table: You can't flank with a ranged weapon. Plus, of course, it's not even available until 12th level. (Rogue hits +6 BAB at 8th level, so Manyshot happens at 9 and Greater can happen at 12th.)
Now, is it "broken" for a 9th level character to be dealing 45 points per round? Hardly. a well build melee type can deal that much fairly easily: Two handed weapon with magic, a decent Strength and power attack. PA for 3 sets the attack rolls even (+6 at level 9) gets +6 damage per hit, on two attacks per round, and STR x 1.5, on top of whatever the weapon does. Guessing an 18 Str isn't unreasonable, so 2 D6 for a Greatsword, +1 for magic, maybe a Flame of Shock dice like we gave the Rogue and Scout for another D6, +12 for Power Attack and STR. Do that and you're going to average 23 or 24 per bhit, just like the Scout, so numbers in the mid 40s aren't unreasonable. And that's not even counting the feats like Weapon Specialization (not available to Scouts, Rogues or Rangers). There's another 2 points per hit.
Compare to spells like
Scorching Ray at the same level: 28 points per round average, no Save.
Fireball or
Lightning Bolt? 32.5 per shot, with a Save for half. If it weren't for the AoE aspect, we'd be calling the spell caster's under powered.
