Prius cell talking points, in my opinion (and in no particular order), are charging speed / convenience, deliverable power, form factor, known quality, life span, and durability. The charging method is just "different" than folks are familiar with. I'm really trying to emulate (not duplicate) the 2 principles adhered to by Toyota. In plain talk;
don't drain em' and don't stuff em'
I don't believe they need cooling at a 5C rate. The tech specs suggest 10C is no big deal, I'd think that's conservative since they have very little internal resistance and big bolt connectors, but I haven't tested them continously at that mark.
My standard ride is a 1000w BMC motor on a midengine - through - the - gears running through a PowerPack controller. From previously using a watt meter on a lesser ride I know the motor easily pulls 1000w at 44v* (20+amps - around 3C) without the batteries getting warm (I don't drain them - end of discharge is where you see garden variety cells heating up).
I think you'd be fine in parallel - good luck if you ever hit the peak 10C rate of >200A! They'd be pretty easy to hookup to a bus bar, you'd want a big fat cable to the controller. I do something similar for my electric boat which can hit > 100 amps when I'm showing off. One thing to keep in mind is finding matched cells - you'd want a complete Prius battery. A Gen III prius pack has 28 cells - you'd get your 4 x 6 pack that way (send me the two left over
Charging should be no big deal, at least with the charger setup I use which is 1 charger for every 2 cells (all matched for output to within .01v). Check your AC circuit to make sure you can pull the total wattage though. Natch, without a LV cut off you'd need a comfortable way to do health checks on the cells from time to time.
An 8 amp charge rate (>1C) seems aggressive to me if they were charging a big string. Although they have little internal resistance I'm much more comfortable pushing charging limits when I'm hitting individual cells rather than feeding top amps through a battery string. If your friend was relying a commercial charger he was in trouble as soon as he plugged them in, regardless of the amps IMHO.
By the way, thanks for all the personal emails and interest - since there's quite a bit of duplication in questions I just posted the answers / comments here.