YES, probably might be superfluous! ;-) Please, set all your other impressions aside. ;-)
Math, it's always just math. An increase in voltage (48V-52V) is not necessarily an increase in Amp hours (Battery Capacity).
I agree with ... As posted above:
Basically, Volt = top speed (Higher Voltage = Increased TOP SPEED) 48V (54.6V fully charged) to 52V, not so much.
Ampere = torque (More TORQUE = EASIER CLIMBING, and gets to top speed QUICKER) What YOU WANTED!
I will add: Amp Hours = RANGE (How long your battery lasts ... capacity)
IMHO: Increasing your controller amperage will reduce your range, but produce greater torque, with a very slight increase in top speed.
If you can live with the decreased range of the 35 Amp controller and your current battery, great. In order to increase your range you will need a battery with more Amp Hours. 48V or 52V, pick one.
A Formula for you: Volts(Battery) X Amps (Controller) (not amp hours) = Watts
Example: My Sondors MXS - 48Volt (battery) X 25 Amp (Controller) = 1200 Watts (power delivered to the motor) At 100% Full throttle or PAS set to100% Here's some What If Numbers:
52 Volts X 25 Amps = 1300 Watts
48 Volts X 35 Amps = 1680 Watts
52 Volts X 35 Amps = 1820 Watts
Simply plug your numbers in the above formula to see the results of any planned changes or current settings.
You have many choices available to you , depending on your preferences. I would probably NOT upgrade the motor or battery until they die. To increase the range of my Sondors (750 Watt) MXS, I run a dual/parallel battery setup. Each is 48Vx17.5 Ah. I run one battery until it's nearly depleted, then engage the second battery. Twice the fun. For easier hill climbing (I pedal), I swapped out my factory 48T chainring for a 42T and will soon be changing my 14T-28T freewheel for an aftermarket DNP 11T-34T one. That should let me cover almost any situation I come across. Good luck.
Also excellent; thanks very much for your input!!!