Next consideration - the diagonal of the garage space is longer than the length so you might get say 9m hull diagonally.  

Another consideration is to build sections in the garage and permanently join outside the garage.

The paper linked here has data produced under controlled conditions for the USN for surface finish versus drag:

Table 2 gives the increase in drag for various surface roughness at 15kts for a frigate.  The Reynolds number will have some bearing but close enough for a 10m hull doing say 12kts. This table correlates well with my personal experience although only qualitatively; meaning no hard data. 

The 2% extra drag for a freshly anti-fouled surface becomes less than 1% loss in speed under powered conditions.  Speed loss could be slightly higher percentage when going to windward as overall efficiency is key to windward ability.

Any carefully applied two part paint will be considerably smoother than freshly applied  Anti-foul so there is potential to have much less than 2% increase in drag due to surface finish with reasonable care.

A small step in the hull is not the same as surface roughness over the entire surface.  2m from the bow the flow will be turbulent under most sailing conditions for a slender HP hull where pressure gradient is low.  At any reasonable speed the boundary layer thickness 2m from the bow will be greater than 10mm so a 1mm step for example will not cause measurable increase in drag.  At higher speed a step might result in flow separation that could reduce drag if surface air can travel down behind the step to create a wedge of air - a tiny reduction in wetted surface.  Stepped planing hulls  make use of this but with big steps to significantly reduce wetted area.

My guess is that a step greater than 5mm would cause a measurable increase in drag under typical sailing conditions.  

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