most electric motorcycles seem to be quite slow.
Which ones have you looked at. I've seen quite a few that would do over 70mph, some up to ~100mph and worked on the Motoczysz bike that could go over 100mph. Look at some of the TTXGP bikes.
What happens if you need 400A and contactors only seem to come with 200A continuous? can you run them in paralell?
200A continuous, but much higher peak currents. DO NOT put them in parallel, not a great idea if one fails. You will likely not EVER see 400A on the battery side except during really heavy acceleration.
I'm not keen on LiPO because batteries are a disposable item, thats a LOT of money to spend on disposable stuff. I realize if you factor in max charge cycles etc they're not bad, but still, its not in the budget.
3 times the life, 1/3 the weight and at a cost that is about $1.10USD an Ah and dropping. They're disposeable, but you get more range, which adds value as well as life (if you don't use 100% of the batteries every time you go out).
When I've looked at deep cycle batteries, the manufs don't list anything like the CCA, (often only capacity) so how can you work how much current you can draw from them for x amount of time?
download the datasheet for that battery. search google for "xxx model battery" and "pdf" and you should find it. that'l give you max discharge rates and hopefully a discharge curve.
Then ofcourse how do you work out how much capacity you need?
You said you don't care about range? range = capacity essentially. If you want more range, get higher capacity. If you just want high power, get some stout batteries that can handle the discharge rates you need. Power and Energy are two different things.
Lets say you have a 100Ah deep cycle battery, is it realistic to think you can get 200A from it for 15 mins? or 400A for 7 mins? (50% DOD)
Again, look for a datasheet on the battery.
Imagining 6 100Ah batteries on a bike with 50kg of motors is no joke 
Then how do you work out range at a conservative power output and speed?
Are there any online calculators you recommend?
motorcycles usually get around 100-150wh/mile (150 being really high).
take your pack voltage, and batteries (lets say 72V and 50Ah for 3600Wh). If you are conservative, you could get close to 100wh/mile cruising around 45-50mph. With lead, you'll get maybe 50-60% of their output (due to peukert effects and inability to go past 80% without killing them) so you're useable Wh is MAYBE 2160Wh. Then controller and motor inefficiency, you'd be looking at maybe 1900wh or less. So that puts range (with lead batteries) around 15-20 miles.
100Ah batteries are not light, nor are they very small, it'd be hard to fit that on a small bike with 2 motors.
You need to be asking yourself these questions:
What is your budget?
What range do you NEED?
what top speed do you NEED?
What top speed do you want?
what kind of acceleration do you need?
