Why not subtract it on arming to have zero?
Altitude displayed/used is relative to the Home location and Home location can be set away (not the same altitude) from take off location.
Is just waiting for more sats the only way to have it more accurate?
VDOP give a value related to GPS altitude accuracy which is not directly dependent from the sat number.
If you want to ignore altitude correction by the GPS in long term you can try setting:
RevoSettings > BaroGPSOffsetCorrectionAlpha to 1
Please note 16.09 do not set the initial BaroOffset value (after a filter reset) very precisely so you will expect a difference on altitude displayed, even if Home location and take off are in the same location.