They will get worse after time... as they become more brittle and let more oil past them
Other possibilities are worn rings (as said above) you would tend to find that the car smokes (blue) throughout the rev range and especially when under heavy load.
Of course turbo is a poss but unlikely as you say the smoke goes away after a while. turbo would normally only smoke on over-run (ie after you back off the throttle) pouring blue smoke out the back
To change the rings is... erm well not easy! the sump will have to come off and dependant on wether the car is 4x4 or not then it can be time consuming.
You will then need to remove the pistons and re-ring them.
This in itself is not guaranteed to cure the problem as the bore itself may be worn and ovaled therefore letting oil past the piston anyway.
Best option it to change stem seals and check bore condition (obviously with head off!) if there is a big lip at the top of the bores then i would say oil is passing the rings this however should be unlikely as you say the car has been rebuilt recently"!
CheeRs
Phil