Yes, XFire is correct.
Negi with only one layer of thousand bolts = able to do a 150Km/s instant movement. Weakness is that it is "telegraphed", at least for those experienced enough to being able to react to it.
Negi with two layers of thousand bolts = Now he constantly moves at the speed of 150Kms. Both his body and thought processes are accelerated. The previous weakness disappears.
And like XFire said, some of the high end people like Eva with Queen of Ice ME, while not being able to move as quick as him, was able to react to his speed.
If anyone wants to remember more about Negi's ME, here is the explanation taken from end of volumes and translated by CanonRap:
Spoiler for a bit long:
He Astrape Huper Ouranou Mega Dunamene [Η Αστραπη Υπερ Ουρανου Μεγα Δυναμενη]
雷天大壮 raiten taisou
Supplement: Khilipl Astrape
Great Vigour of the Thunderous Sky. By absorbing the most powerful spell in the lightning element, the mage physically transforms into sacred fire – that is, a charged particle mass, which grants an instantaneous speed of 150km/s: Lightning Speed Instant Movement (雷速瞬動 raisoku shundou).
However, a major disadvantage exists in that being truly “lightning,” a positive streamer will always move before the mage, telegraphing the destination of every movement. Also, the speed boost is limited to the mage’s physical actions.
Da Zhuang (大壮 taisou) is title of the thirty-fourth hexgram in the I Ching, thus named because the hexagram composes of ‘Force’ (‘Heaven’) and ‘Shake’ (‘Thunder’). The hexagram itself more or less means ‘strongly, intensely, lively’, thus the Greek name: “lightning above the Heavens is extremely powerful.“
Tastrapa Huper Ouranou Mega Dunamena [Ταστλαπα Υπερ Ουρανου Μεγα Δυναμενε]
雷天双壮 raiten sousou
Supplement: Khilipl Astrape x 2
Twin Vigour of the Thunderous Sky, Mega Dunamene 2. By absorbing the ‘Thousand Thunderbolts’ twofold, the mage can maintain a constant speed of 150km/s – Constant Lightning Form (常時雷化 jouji ikadzuchi-ka). At the same time, this form increases both physical mobility and mental processing speed, allowing the mage’s mind to keep pace with a “lightning” body.