DC PWM Control - Higher voltage=More efficient?

In general, higher voltage and lower current (for the same HP rating) will result in lower I*I*R losses. Switching losses tend to increase with voltage, but not as quickly as I*I*R losses decrease. You may find an efficiency "sweet spot" at 36 or 48 volts, but there are other practical limits.

I ran a few generic loss calculations and believe you are NEAR/AT the limit of what your system will provide. About 900 watts of power is needed to maintain ~24 mph on level ground. This power is used to mostly overcome aerodynamic drag, but some mechanical drag and electrical losses are also included.

NO amount of voltage or gear fudging will significantly increase your top speed. If your goal is to go faster, you need to re-evaluate the overall design using aerodynamic drag as the #1 power loss. Significantly reducing drag is possible, but not very PRACTICAL on a simple bicycle conversion project. More batteries(energy) and more powerful motor and controller will be needed.

I presume that you would be replacing the motor to accommodate the higher voltage, otherwise I am very certain you would burn the motor out. For the same power motor at 36VDC, the short answer is yes you would slightly increase the power efficiency. However this would not be enough to make a difference of more than a few milliwatts of power improvement. The power companies use something like 10KV to over a Mega Volt to reduce wire losses in power transmission. Still it is an interesting idea. Increasing the connecting wire diameter would also do about the same thing when considering a two to one or so voltage increase. If you were going to improve the batteries and the motor power then you could make it an overall improvement worth doing. I don't know if your bicycle motor charges your batteries when you are coasting or going downhill but that would be a possible consideration if you wish to increase efficiencies. Best of luck on your project.