Will using a 15 foot usb repeater, rather than a 15 foot use cord, give more power to my wireless dongle?

No, with a USB device, it either works or it doesn't work. Using a powered USB line instead of a pive line won't make a difference. That's like saying, if I plug my cordless phone into a surge protector, will it have a longer range? No. The technology has a limited range that is independent of available source power.