Dear Lawrence,
Extra exposure alone should do it. As little as 1/3-2/3 stop. If your negs print OK on grades 2-3, leave the dev times and agitation where they are.
Increased agitation = increased speed at a given contrast + slightly increased contrast for a given time + slightly lower sharpness.
Reduced dev. time = reduced contrast and (usually) reduced speed + slightly higher sharpness. In other wirds you'd be pulling in two directions at once.
The film and dev you are using would normally be chosen for tonality (if at all) rather than for speed.
Cheers,
R.