but I would like to be able to push it 40 mph if I have to for short bursts.
This is what I replied to. I wrote just about any setup can be configured to do it.
In english we are talking peak here. Unless you are a terrible builder or abuser it is very easy to do.
The 25-30mph cruise should be easy, and honestly anything above that and the danger levels rise exponentially.
More amps will mean higher stress(torque) on the drivetrain plain and simple. On a 72v system you can still get the desired wheel torque by using a higher cog on the rear. But if you use a low voltage high amp setup, all the torque is there from the motor shaft, and goes through every component, including the internal reduction gear(hence bbshd blowing up at 52v 60A but doing fine with 72v 40a).
Aside from that, let's talk about the battery itself. You are building a 180 cell pf battery. So it should be good for 3.6*10*180=6480w peak. This can be entertained with a 90a controller at 72v. If you ever used this battery with a different motor, this is all good and reasonable on both numbers.
With a 52v system, you would need over 120a to make use of your 6.5kw battery, which leaves you with much fewer and more expensive options in terms of BMS, controller and motor. To extract 5kw peak you would need 96A from your controller and the 3kw cyclone not handle it realistically, so it would be untapped performance, forever. With 72v only 69a is needed for 5000w, many people run this setup.
I would certainly not leave any performance on the table.
I think you know even the stock controller is capable of doing 72v, so with the 40a one, you would be having 2880w instead of 2080w from day one.
In practice, if you aren't interested in shifting before and after every stop light, a motor with a broader power range is more hub-like, far better to commute with, which is what the 72v setup gives you. Yes, you can do the same on a 52v bike in a lower gear with a higher amp controller, but you would be changing the drivetrain parts more often.