Normally, you can expect to gain an 'effective speed' with Rodinal by simply increasing the development time; the contrast will not rise significantly, unlike most developers. But HP5+ is the one film Agfa doesn't recommend with Rodinal at 1+50, and my (limited) experience bears that out.
So, if you want to stick with Rodinal, you can go with Tri X or Neopan 400.
Or, if you want to stick with HP5+, you will get excellent results with Xtol or DDX. In either case, you will get as much of a speed increase as you wish, and fine negatives as well.
There are subtle controls you can make use of: increasing the development time while reducing agitation to every fifth minute, which will give you a shadow speed gain with normal highlights; using XTOL as a seasoned, replenished developer, which is dependable and adds local contrast while retaining a normal scale, and so on.
DD-X, I understand, works in a similar way, although I have no first hand experience with it.
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