Flight times will depend on the Amps you draw, and how you manage your throttle. I would imagine that with a total of 4200mAh at your disposal, and a peak current of ~35 Amps, you can easily run 10 minutes or more with throttle management.

Weight is pretty easy to determine. The weights of everything being compared, glow engine, motor, and battery, are posted on their respective websites. Also take into account a full load of fuel, throttle servo, tank, etc.. My money is on the conversion coming out pretty close, if not a bit lighter than the glow version with a full tank. (Note: 8 fluid ounces of fuel does not weigh 8 ounces)

3.7 pounds for the .46 size eh? Sounds like you build light in the first place.