It’s all relative. The value of 27,000 Avios, and the relative values of J and F, will be different for different people.

Personally I wouldn’t bother as I think I would get better value from 27,000 Avios with RFS redemptions within Europe, or for short haul on CX or JAL. I do, however, have the benefit of being OWE, so I get many of the “soft” benefits of F regardless of my cabin of travel. I also don’t think there’s a particularly huge difference between BA J and F on a night flight.

Your travel patterns and preferences may vary.