Clive,

I think you're right to let experience speak, and let's not try to fix what isn't broken.

But the OP asked about an improvement, which is what we should try to accomplish to help with.

It may or may not be that the amount of developer used by OP is enough according to Kodak's instructions.
Irrespective of that, OP has stated that the use of Grade 5 filtration has helped in printing, which to me suggests underdeveloped negatives - for what the OP is trying to achieve.

To gain an improvement, and to change as little as possible in their work flow, to develop the film longer makes sense, because it would enable similar print quality with less printing gymnastics.

I always say that the paper and developer combination used by the printer, is first target in how film should be exposed and developed, in order to get the results that we want. To see the potential of US photographers controlling the whole chain of events is tremendously empowering. When you learn this, printing becomes a lot less frustrating, and with much less darkroom waste as a result.
But it does take some hard work and an objective view of our own results, in order to get there.