There is no set "Efficiency" associated with these gears. The efficiency is really a system measurement. Systems that use helical gears are less efficient than systems that use spur gearing due primarily to the fact that helical gearing produces an axil force in addition to a tangential force and radial force produced by spur gearing. This axil force increases the friction between the gears themselves and is translated to the bearings that hold the axils in place. The bearings will experience a much higher axil load in a helical gear system than in a spur gear system.
It is important to know that the helical gears should be kept lubricated. Running these things with little or no lubrication means a dramatic drop in system efficiency. It's also important to know if the bearings that support the axils can withstand the significant side loads (axil forces) that are generated. Make sure that these bearings are also well lubricated.
Roughly, efficiencies for helical systems can typically run in the 50-70% range. Spur gear systems run in the 80-95% range.
Here's a good site for some data
QTC Gears