Gear Second has been explained. The pumping of blood through the vessels at ungodly rates, expelling steam as an aftereffect and strengthening the body immensely, is Gear Second. No normal human could do this without having his body break apart, simply because their blood vessels would rupture from the strain.
Taking that, we also must consider Luffy's own words: "I'm glad I met you guys here." Saying that he saw the tenfold kick and got the trick down seems to indicate to me that he sort of bastardized Soru for his own purposes. It isn't as masterful as Soru, but I assume with Gear Second he's able to accelerate and keep awareness at a level equal to or surpassing the CP9.
Because Gear Second is so volatile, it's naturally hard to maintain. Even if Luffy is exceeding Soru's speed in it, he can't keep up with the loss of stamina yet. Strength, speed, everything increases but the vitality… Which is why Luffy couldn't beat someone like Lucci in that state.
Evidence... The same kind of shape was created when Luffy moved during the Blueno fight as I recall, a warped white spot. Luffy himself said he got the trick down, and Lucci has already offered his explanation (which Luffy failed to refute).
And for the record, if Luffy does, indeed, "have the trick down" for Soru, then I don't see why he wouldn't be able to use it in his normal state. What Octogon's offered for evidence, however, might be considered artistic and spatial decisions.