Those calipers are fine. The design of the leverage arm lengths of the caliper gives them the same clamping power as the 105's that you are considering as substitutes. So there is no gain to be had by doing that. But I can tell you from personal experience that the stock Tektro pads suck as badly as stock Shimano pads. Getting replacement pads may not seem very high tech but it truly is what you need to make these work. I'd suggest you get the full Koolstop salmon pads instead of the dual pads. From there correctly align the faces for maximum contact at the time of installation. The pads have concave washers to allow altering the angle over a range that should be wide enough. From there it's just a case of correctly centering the caliper before the final tightening of the tubular nut onto the stud. Final slight trimming is then done with the centering screw adjuster. But the bulk of the centering should be done by properly centering the caliper in the first place.
On clean rims the salmon pads offer the most grip for a given lever pressure that you'll find. If the brakes still suck then there is something else at play here. For example if this bike has the super cheezy "modulators" in the brake lines those need to be removed and tossed into the trash. The lever should have a very firm feel and not be able to bottom against the grips. The cable clamping point then being adjusted to make the engagement point of the lever occur at a distance from the grip where your wife likes the feel and can generate adequite finger pressure comfortably.