[PaulF]

Here is my take on this situation:

Very few root systems go below the 18" level. Those plants with taproots extending farther than that are searching for moisture rather than nutrients. Even most trees have root systems that are primarily in the the top 18-24 inches in the soil.

Tomatoes, as with most other garden plants, have besides the primary root, secondary roots and root hairs that extract nutrients and oxygen. All of these systems are relatively close to the surface.

I don't think putting compost down that deep would hurt, but I see no real value unless you like to get the exercise that comes with digging holes. Most soil tests for soil health say to get samples in the top 12".

Lots of folks would love to have 18" of good soil and they grow tomatoes on less than that.