The title basically says it all.
In my very limited experience…
Increasing your wattage limit seems to increase torque substantially, given your motor max is set high enough, and your battery max can provide the desired wattage given your battery voltage after accounting for voltage sag (batt max amps x voltage after sag = wattage).
It doesn’t seem to affect top speed. I set my wattage limit to 20 and still reached top speed eventually (I was in a tuck). Same with 150 watts, 660 watts, 1440 watts… It just increases torque if your settings are set appropriately.
To go beyond my answer to your question…:
I am in the midst of a conundrum personally, related to your question. I want the most watts to get torque, but don’t want to sag my 25Rs too much, or substantially kill efficiency/range. I can set my wattage limit to 150 per vesc and get up to speed with great efficiency (15wh/mi), or pump it all the way up to 1440 Watts (25-28wh/mi) which is much less efficient but equally more fun, or set a more sensible but still powerful 800+ish (19-21wh/mi) and kind-of have the best of both torque/speed/efficiency. All this, considering my setup could hit 1440w on each vesc / 2880 watts total on a full charge after accounting for sag, but that means my total range takes a nose-dive/dive bomb into oblivion, quickly backing off to the more sensible wattages thanks to discharging cells regardless.
I think a sensible median for me will end up being 500-600 watts per motor, averaging 18-19 wh/mi… both efficient-ish and fun with enough torque, and top speed is beside the point… just takes time to get to speed based on torque from the wattage limit set (if set appropriately, again).
Wattage has a more direct relation to torque and speed compared to voltage and current. That’s what I’ve learned. 
There already has bin a topic created about this,