What type of oil are you using?
Not that it is the cause just asking

wondering if there is some type of additive in the oil that burns off as it warms up,
There should never be any spitting out the breathers if the engine is in good shape,
do you run a PCV system?

I would do a leak-down test, it will show problems not identified by a compression test.

is it smoke or steam???

maybe you have alot condensation in motor, when gets hot , pushing out breather.
but it looks more like smoke??

You're absolutely sure the oil looks good? No fuel smell, no white streaks, foamy, changes in viscosity? What ever this is, it has to pass through the crank case. I had an old Pontiac 389 that the valve guides were so bad the engine wouldn't run with the valve covers off and it didn't smoke like this. Puffing indicates this is a combustion related issue or at least related to one or a few cylinders not all. Is this a closed cooled motor or seawater? If you have through hull exhaust watch your exhaust closely, have someone follow you and see what your exhaust is doing. What temp does your motor run at?

odd it was fine the first few times out, you may have cracked a ring, or somehow created this during the breakin ????
please keep us posted!

Not necessarily. If it is an oil ring it usually will not affect compression. A leakdown test would be next.

So did you build the engine or somebody else ?