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Is there any way to make siphon speed last longer (what should I slot it with to do so?)
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Nope, it has a set duration. You can slot it with Recharge Reductions in order to make it more available, and it does stack for extra speed.
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Does slotting knockback into energy attacks make it happen more often?
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Nope, it just increases the amount of knockback that happens when it does connect.
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Can siphon speed replace a travel power, or should I get one and then respec into inertial reduction at level 28?
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I got my Fire/Kin to level 28 with using only Siphon Speed, then I picked up Hover at 28, adding 2 extra slots at 29. It's frustrating when Siphon Speed misses, though.
@Synaesthetix
"Here, take some more bees with you. You may need them."
Union: FU//LoUD
"that Syn is that that" - Mothers Love
Thanks for your prompt response. 
To be honest, I still dont have a travel power, I live on siphon speed and Inertial reduction. Tho if you recharge slot IR enough, you can have more than 1 running at a time, for a super super jump, really is alarming how far it can take you. As for accuarcy, got my speed boost triple slotted for acc, mainly for pvp, so is very rare it misses.
Slotting Siphon Speed for accuracy and recharge is the way to go (for me anyway). I've never had travel powers on my kin/en defender and feel no lack. Though there was a stage where I just accepted that if I missed, I ran along slowly until it recharged.
And it does make you mighty unpopular in pvp, when you get your travel power by stopping someone else flying, running, jumping and in the case of heroes probably skipping fast.
Right, most questions have been answered 
I tried to survive on Siphon Speed for the early lvls, but soon got super speed. I thought I would get Inertial Reduction to see what it was like. One word: YAK! I hated it. Super speed was too tempting to use with Siphon Speed, which completely messed me up. So I respecced to good ol hover and TP. I'm still the fastest in the west 
Is there any way to make siphon speed last longer (what should I slot it with to do so?). Does slotting knockback into energy attacks make it happen more often? Can siphon speed replace a travel power, or should I get one and then respec into inertial reduction at level 28?