If you just want 1000w for 40 ah of range then a 2c pack is fine. For a better chance of getting it's full lifespan, plan for discharging it at 1c for the most part. As cells age, or if you just get a few weak ones, the extra amp capacity prevents the weak parts of the battery going to 3c discharge, which would kill it quickly. Once you lose some capacity, the c rate starts climbing compared to when it was new.
I just advise the pouch type packs because really big round cell packs tend to have a bad cell or a bad connection lurking in there. Often as not, a really big one suffers damage in shipping, and never puts out the full capacity.
I was assuming that like many of us, your interest tends to be toward the next bike having more watts than the last one. Emissions free is one source of a123 pouch packs, and lately there are others. I don't have the info on the tip of my tounge where else though. There are threads that talk about sources though, in battery, and for sale sections.
RC lipo (lico) is the other source of high c rates. If you can afford it, 50c is out there, and more! But 20c RC lipo is fairly cheap. Less cycles so only cheap up front. It's the way to go for high power with light weight, such as racing and dirt riding where handling is far more important than anything else including cost.
A 40 amp controlller (2000w?) has a lot of potential to ruin your current battery. When you do actually pull 30 amps, you are getting really close to the limit. But very likely, when you cruise at full throttle on flat ground, you pull closer to 15 amps, and actually discharge at .75c 90% of the time. But if you have long hills, you could be pulling 30 amps for some time.
I would REALLY discourage you from buying a 45 pound battery that is in one piece. You better strap 45 pounds on your bike and ride it before you decide it's OK. My commuter can carry 50 pounds, but the handling gets worse and worse as you load it up. Two batteries that carry low in panniers will help, but you will still need a good quality bike with lots of side to side stiffness. A cheap alloy frame bike loaded with 50 pounds will display an amazing flexibility from side to side. They call it death wobble, or tank slapper. The way a bike is designed, it really wants the weight to be located on the seat or the pedals. 50 pounds elsewhere bends the crap out of weak frames. Carry as much battery as possible in the frame triangle, which locates the weight in the middle, rather than one end of the bike.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.