I never thought the Health Regeneration Implant to be worth the expense and giving up another more useful implant.
However, the effects should stack like all other Perk applied bonuses do.
It is horribly weak, the only benefit I've seen was on hardcoe over a long period of relative calm after a firefight, I tended not to heal unless below 66% or in a dangerous area, so it saved me a sunset sarsaparilla, but then on hardcoe you eat enough food and drink enough water to heal up over long periods anyways...really weak implant, worse than any SPECIAL and definitely than sub dermal....and it costs more... : /