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Walter --
I think you can stretch the dilution limits a little without too much concern. The manufacturers tend to recommend very conservative practices, and that's understandable; after all, they want to be sure that their products are perceived to be reliable under all conditions of use.
I don't believe you'll have any serious problems with moderately increased dilution either in the Jobo or the tubes. Some time ago I ran developer dilution tests using HP5 Plus and TXT in D-76 (straight, 1+1, 1+2, and 1+4), and TMX and Delta 100 in Xtol (same dilution range). Naturally the test development times had to be adjusted for the various dilutions. All the tests were done with 4x5 sheet film in the tubes (2 ounce capacity).
The summary of these tests was published in The View Camera Store's quarterly newsletter, D-Max, volume 5, issues 3 and 4, 1998. If you're interested I think those back issues are still available, and I believe they have an online version. Check with Dennis or Fred @viewcamerastore.com.
In a nutshell, the data indicate that the dilution ratio had almost no affect on the maximum gradient attainable, and in D-76, at least there was no significant loss of effective film speed; in fact, when there was an apparent speed change it suggested that moderate dilution actually increased speed by a fraction of a stop.
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