SNG brakes are fine. Tektros tend to be a little lighter and more expensive for about the same braking-performance. With both, you'd want to switch to Kool-stop pads for maximum-performance anyway.
The point of highest liability in brakes is with the cable-fixing bolt. You MUST clamp down the cable with precisely the correct torque. Too loose and you risk the cable slipping under braking. Too tight and you risk crimping the cable and having it snap under braking. Or you may overcome the bolt's yield-strength and it will have little clamping-force. Make sure that bolt is secured properly!!!
After that, braking-power with any brake is determined by the front-tyre's friction limit (provided the rider has enough skill to built deceleration-forces up to that level). If you desire maximum safety for you and GF, then I suggest you practice maximum-braking in a parking lot until you can decelerate from 20mph in 17ft or 30mph in 34ft. That's pretty close to the physical limits of the tyres traction.
Practice building up force on both levers quickly, but don't grab it all at once. It takes about 0.2-0.25s to built up to maximum deceleration. During which time, you want to scoot your butt off the rear of the seat and rest your belly on it. This shifts the C.O.G. rearwards and downwards to fight the weight-transfer to the front and prevent the rear from lifting and throwing you over the bars. As deceleration builds, you'll want to release pressure on the rear brake (while increasing the front) to keep the rear right at the edge of lock-up. There's been plenty documented cases of cyclists and motorcycles sliding into a car with the rear brake locked up and skidding. If they had used more front-brake and not locked up the rear, they would've stopped in plenty of time.
As another test, you can measure stopping-distances from 20mph with front-only versus rear-only brakes and compare to the technique I outlined above.