Antenna: Impedance matching vs. tuning
I have measured an untuned IFA to have impedance of Zin = 7.7+j90 @
  865MHz


The 7.7 ohms "real resistance" measured represents both the radiation resistance AND the antenna losses so, if you can adjust the antenna length to give you a higher value of overall resistance (and therefore a higher figure for radiation resistance) you will get better antenna efficiency and more easily match your output driver (50 ohms maybe) to your electrical radiation resistance. 

You should also find (that with a simple dipole or quarter wave monopole) that the reactive component becomes smaller and therefore you more easily can dump power into the antenna. Here's what a monopole impedance looks like: -



At a quarter wavelength (optimum for a monopole) the reactive impedance becomes zero ohms and the resistive part (for an ideal monopole) becomes about 37 ohms. If the antenna is significantly shortened the real resistance rapidly approaches zero so, if there are real antenna losses (as always) any power pushed into the antenna is mainly wasted on heating the antenna and a significantly smaller percentage of power gets radiated.