Circuit for overdrive on vibrotactile motors
I used to work in the vibrotactile transducer industry (no not the adult-entertainment-toy side) and some of my work was with those little eccentric-mass motors (the rest was with voice-coil type transducers).
I can definitely confirm that a little 'overdrive' boost at turn on gets the motor up &amp; spinning quicker and helps to give you more recognizable haptic patterns with sharp starting edges.
In my work I always had a supply voltage available which was higher than the rated voltage of the motor and I used PWM to control the motor H-bridge driver circuitry - this allowed me to easily overdrive at turn-on by providing a high duty-cycle for a few milliseconds before dropping down to a lower value for the run period.
Something you haven't mentioned, but can be related to those nice sharp starting edge is what happens at the end of your pulse - if you just turn the power to the motor off it will take its time as it spins down, and depending on the nature of your haptic patterns this can muddy things a little - particularly if you're trying to produce some short, sharp pulses.
An H-bridge driver for the motor allows you to put it in to 'short brake' mode when you want it to stop running and this makes it stop much quicker than just letting it spin down at its own rate.
With this in mind, I'd highly recommend that you consider the slightly more complicated route of using PWM and an H-bridge driver for your motors.  

But, if the additional complexity is off-putting, here is a trick which you might be able to use - but you still need a supply voltage higher than 3.3V - you could probably use the 5V pins on your Pi's GPIO header for this.  







simulate this circuit &ndash; Schematic created using CircuitLab

The values of Rs and C1 will need to be tuned for your application - I would start off choosing Rs to have a value of about 1/4 of whatever your motor's resistance is (measured with a multimeter). You should then fine-tune it to get about 3.3V (or whatever your motor is rated for) while it is running.
C1 should be chosen depending on how long you want your overdrive to last.
R1 should be chosen to be low enough to ensure that the transistor switched on hard (saturates), but not so low that you're pushing excessive current though its base from your Pi's GPIO.
You would achieve better performance replacing the NPN transistor with an N-channel MOSFET and removing R1. Make sure to choose one which is specified as 'logic level' to ensure that your Pi can reliably turn it on.
How the circuit works is that before you turn the Pi's GPIO on, the capacitor charges up to 5V through Rs and this 5V is then available to 'overdrive' your motor as soon as the GPIO turns the transistor on.
The length of time the overdrive lasts will depend on how much current the motor draws and the size of capacitor you use.  

Some drawbacks of this circuit are that it wastes power in Rs and you have to wait a short time after turning the motor off before you turn it back on again to allow the cap to recharge.