As your paricular experience proves, the D4U1 upgrade (just like many upgrades) is not a "one size fits all" type of deal. Some people notice a change right off the bat while others do not see much improvement at all. In actuality, there is
very little difference in the amount of line pressure that is commanded between the H3Zx and D4U1 programs. The D4U1's real advantange over the H3Zx lies instead with it's upgraded chipset and reworked transmission management logic, both of which contribute to the feel of "better shifting".
There are several options, including an LPM from Ted Breaux, an SCT chip from Doug Lewis or a chip from myself. If a chip doesn't suit your fancy, then you can go the more hands-on route of controlling the EPC directly via a pressure riser, like this one: