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Larry --
320 TXP, like the older TXT, doesn't have great development latitude so it's probably unrealistic to expect it to reach extremes of contrast. But DDX will do about as much for it as any developer can and it's reasonable to use it at 1+3 or 1+4 for high contrast results. Also, dilutions of 1+11 provide useful curves so the 1+15 dilution you propose may be practical, too. How high contrast do you need? Average gradient numbers are more meaningful than N-numbers for expressing the film's capability.
The Contrast Index method for placing the curves' speed points relates speed point density to curve gradient with two arguable benefits. First, it tends to keep image gradation relatively uniform; and second, as you mention, it conserves film speed (although it's more accurate to say the 0.1 over B+F method wastes speed) when curve gradient is low. If using the CI speed point location results, consistently, in thinner negatives than you like you can increase negative density and retain the other advantages of the CI method by simply adjusting (reducing) the working film speed; or, if you're using Expo/Dev, by using the Correction feature on the first screen.
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