How are you planning on getting higher voltage from your cells without a step up regulator?
And are you sure you need higher voltage? Mechanicals aren't really intended for that use. That's what VV devices are for.
To answer your question as accurately as possible, more info is needed.
Basically at the end of the day what really matters is the wattage.
Wattage is contingent to not only the voltage supplied but also the current which in its turn is directly proportionate to the resistance in your coil(basic ohm's law).
So now the real question arises. What are you willing to vary to get higher wattage?
If your coils are preset/prewrapped(you buy them that way or always wrap them to exact same resistance) and feel that a mech mod on a fresh charge is not enough for you, then more wattage is needed.
This can be accomplished in one of two ways. Either by lowering the resistance in the coil to allow more current to flow through it(it's the preferred method of most mech mod owners) or increase the voltage through the original coil, in which case a VV device is more recommended.
Either way, with basic ohm's law you can find out the current draw on the cell and figure out if you're within manufacturer set parameters. If not, then you probably need to step up to something like the AW IMR which has a much higher discharge rate than most lithium ion cells which are at around only 2C.
Hope your head isn't spinning too much.