How much can you furl a headsail
There is a great difference between simply furling and reefing a headsail with a furler.
For straight furling, either the sail is completely unrolled or completely rolled - and no in between settings that will give any proper shape.
For reefing (you need to have the sail modified for reefing) ..... Typical jib maximum reduction with an installed foam luff will be about 25 to 30%. ... So you will be typically able to reef down to 115 - 110% from 150%. This assumes that the foam luff is correctly designed / tapered and the sail tightly rolled on the foil when reefing. You can also consider to have the luff rope "disconnected" (cut away) at the bottom and top of the sail ... only attached for about 80% of the middle of the luff - to ensure that the center part of the sail tightly wraps to the foil - you might want to discuss this with a sailmaker.
Without a foam luff, etc. you will not be able to ''reef'' and maintain good shape.
I somewhat disagree about performance degradation ... as any performance degradation WILL affect the boat''s safety. For instance (and especially without a foam luff or other means to control the SHAPE of the sail, etc.) as the wind rises and you simply furl a little of the sail, the draft will now become extremely full... preventing you from pointing up when necessary; and, when you do try to point up you will heel excessively because of the now too large draft in the sail. Further, you will now find that jib will become extremely unstable ... backing and filling with wind, sometimes unexpectedly - not a ''happy'' place to be.