I know ion pair can obviously change retention time and peak shape of acidic and basic compounds. But does anyone know if strength of signal, peak height and area, would be obviously enhanced or not after adding suitable ion pair into mobile phase for detecting a basic drug?

Peak height may increase as the basic drug's peak shape will probably improve with the addition of an IP Reagent. Peak area should remain the same.
If the addition of the IP reagent significantly increases your background absorbance, then you could see a decrease in peak heights and areas.