Kirito vs the Gleam Eyes (Vol. 1 Chapter 11)
Kirito and Asuna entered the boss room to find the dumbass army getting their collective behinds handed to them. Kirito noted that the Army's collective damage "hadn't even reached 1/3 of its HP".
Kirito's commentary mentioned that Gleam Eyes hitting the ground was strong enough to send one person flying above its head and said boss is this huge.
"Asuna's reckless strike hit the back of the demon while its attention was elsewhere. But its HP hardly decreased at all." - Kirito
Gleam Eyes swung its zanbato at Asuna but she dodged sideways but got flung back by the shockwave of the sword hitting the ground.Gleam Eyes' combination of sheer strength and speed made it so Kirito couldn't even counterattack at all.
And that's the guy whose skill with a one-hand sword able to stalemate Asuna in Progressive.
Kirito noted that the Gleam Eyes' sword style is a modified two-hand style that is impossible to read, and reading into the fight is Kirito's forte.
Asuna and Klein bought Kirito 10 seconds for him to pull out Dual Blades, but he noted that Asuna already lost half her HP just from that.Kirito, finally using Dual Blades, blocked Gleam Eyes' swing with one sword and hit the boss with the other sword, noting that it was reduced noticably.
Gleam Eyes attacked again with a downward swing and Kirito blocked it with his swords crossed (such a block is awkward and stupid if you ever tried it) and managed to unbalance the boss.
"Faster, faster.
The rhythm of my swings already exceeded twice the normal speed, but it felt slow to my heightened senses. I continued my attack at a pace that seemed to overtake even the system's assistance." - Kirito
NOTE: The Gleam Eyes never caught Kirito's blade in the middle of using Starburst Stream so it's anime only, Sword Skills can only be cancelled by something of equivalent force like another of similar level to push it out of its original trajectory. The user cannot cancel it once it's casted.
Kirito, in 16 hits (Starburst Stream) + 1 (switching in as Asuna switched out) did more damage than the entire army (who only took 1/3 it's HP) and Klein and Asuna, respectively high-level clearers did altogether.
I think it's safe to say that Starburst Stream at least doubles Kirito's base attack speed, and amps his damage by a massive amount as his regular swing was noticable, but not really notable, while the Sword Skill ate the entire remaining HP of the boss.
It should also be noted that Starburst Stream and Kirito finishing the fight with a sliver of HP (described as a few dots away from being empty), so it seems that it has massive stamina drain to use, and that Sword Skills actually use up stamina at all.