Depends what your after, shift kit is great for cruising, or as Roger said climbing.
The correct single speed sprocket for engine, rider weight, terrain, ect will likely net a better top speed as 1 chain is better than 3.
39 or 40t is often the best for general purpose, unless you have a worked motor and am after max speed.
I have 1 with 44t, heavily worked and wheel stands at speed.
1 with 39t, relatively stock, quiet, and a nice ride probably does 65 odd KPH
1 with 36t, Fully bombed, does about 90KPH.
and also a bike with a shift kit, fairly mild modifications and will also do about 65 odd KPH same as the 39t.
they all have there own rewards.
But unless you plan to heavily work the motor your not likely to get any more speed from a 36t than a 39, but will lose out on bottom end.