s2000paul

Hi Guys
i have a 99 ap1 with 80000 km
i bought it years ago with 60000 km and it was hardly used for 8 years
when i got it from the seller i flushed out all the oils and there was a little bit of swarf in the rear diff oil and it was very very black
i put in redline and its ok when on power but there is some backlash when going on to the overrun and a light whine when going downhill, the oil is clear and hardly a bit of swarf
questions
is this normal
is it worth looking at
what can i do to improve it
i had read in the past that a kia sportage gear set is a valid alternative at lower cost
its this right of just fluff?
cheers
paul