First, if the motor spins the same speed offground as it does onground, at a full charge of the battery, at WOT, then there is a speed limiter in play, and the speed sensor is on the motor wheel.
If it is faster offground (especially if it's significantly faster), then there's probably no speed limiter, unless the speed sensor is on a different wheel than the motor wheel.
If the new controller from a different manufacturer, and the old ones all have the same speed out of the motor when unloaded, then while it's possible they all simply have speed limiters on them, it's more likely that at the battery voltage you've got, the motor itself simply can't spin any faster than that. Increasing the battery voltage would show this; you just have to be careful not to exceed the max voltage of the components (usually the big can capacitors and the FETs).
Remember that the higher the voltage, the faster the speed at WOT, but also the less "resolution" your throttle has: it goes to higher power at lower throttle amounts than before.
Also, the total power into the motor is higher at higher voltages (at WOT, anyway), by the ratio of the old voltage to the new one. If you had, say 360W with 36v, then you'd have 480W with 48v. If the phase currents to the motor are made high enough by this (different than battery current), the motor begins running out of it's efficient zone, and enough more of the power is wasted as heat to be a danger to the motor if you run it like that at WOT very long.
Probably the motor can easily handle the higher power, but it's something to keep in mind.