Don't be afraid to ask. Most of us are quite gentle most of the time!
What you are suggesting is to marginally overexpose the film and then underdevelop it compared to the "normal" ISO rating of Delta 400. As a rule this reduces the contrast of the negative. Why you would want to do this could depend on many factors such as subject luminance, type of enlarger, preferred paper and aesthetics of the final print you are trying to make. The reason to overexpose a negative film (lower ISO) is usually to do with ensuring sufficient shadow detail. There are also those who believe film manufacturers tend to exaggerate the "real speed" of film.
As you are shooting 35mm and are lilkely to be shooting a variety of subject matter with different subject exposure ranges on one roll, shooting at 320 will tend to be a "safety factor" for the shadows. Then using development time for 400ISO may on occasions start to give you some heavy highlights but I don't think it would be a serious problem.
The best thing to do is experiment. Process one roll at 9.5m and another at 8m and look at the difference. Shadow details should be pretty much the same and only the highlights impacted. For the last 5 shots on each roll expose a blank wall at -2, -1, n, +1 and +2 using your in-camera meter and compare. If you can get access to a densitometer you can measure the differences and determine which range is closest to the normal ranges.
This all assumes of course perfect chemical mixing, temperature control, agitation etc etc. Be careful not to be dragged into the search for the magic bullet or follow every new piece of advice you read! Get close enough for your preferred style, settle on a repeatable technique and then just take lots of photos. Only change something once you have perfected one process and then one variable at a time.
