bobones

Here's my $2c.

You're supposed to use BB8681 with FSA carbon cranks: it's different from the normal FSA mega exo BB and different from the Shimano ones. I picked up a BB8681 a few months ago and it came with a spacer, but I understood that to be for MTB frames. From what I'm reading now, FSA triples do not need a spacer, but my double FSA SLK does use a wavy washer on the non-drive side.

I've never had an external BB, whether Shimano Tiagra, 105, Ultegra, FSA, that did not have some bearing friction when installed correctly, i.e. the crank arms do not spin as freely as you see with square taper BBs so stopping after 1 revolution is not unusual IME. They do loosen up a little in use, but the drag is not a concern when riding.

If I were you, I'd run with the Ultegra BB you have now if you've installed the crank to the correct torque spec and:
there is no lateral movement of the crank through the frame
there is no play at the bearings when you move the crank arms from side to side
the cranks spin smoothly on the BB (not necessarily free running)
the crank arms to appear to be the correct distance from the chain stays and do not hit them
shifting at the front appears to work as before without huge adjustment of the FD

I don't see how you can really have a serious problem with this BB if it appears to work correctly as determined by the above. It may not last as long as it should, but you'll do no harm to your frame or crank and you should get at least 1000-2000 miles from it.

Otherwise, pony up for a replacement BB8681.

Replacing bearings seems like too much hassle if you don't have the correct tools to remove and install them in the cups.