Dietmar -
Maybe I'm missing something - - - but there must be a reason why you chose to increase the dilution of your developer. It would seem to me that in most cases, that would then dictate what you would want to do about agitation.
For example, one common reason to want to increase developer dilution is to compensate for higher contrast in the scene at the time of exposure. Increasing the agitation will increase negative contrast, so that would be exactly the wrong thing to do. In fact, to achieve optimum compensation with highly dilute developers, one might actually choose to decrease agitation, even to the point of what is sometimes referred to as "semi-stand" development in which the developing time is extended greatly (perhaps to an hour), but agitation is limited to the first few seconds and perhaps one short period about half-way through development.
Another common reason to want to increase developer dilution is to reduce the size of grain. Increasing agitation tends to increase grain size and would again not be appropriate.