Series doesn't really gain you any torque at all. Motors are rated at the the same holding torque whether series or parallel, and in series, the torque falls off faster than it does for parallel.I'll have to check the calculators but I suspect that I wouldn't gain any speed running it at 60v series over 48v parallel. Yes I'll pick up on torque but I don't think that I'm really short on that.
I agree, though, that 48V wired parallel should be faster.