Well, if you are going to let them have it (the stimpacks/instant heal of health points), just also add the tramatic hit tables.
GM: Lets see, you get hit for 9 points in your left arm.
Player: I grab a stim pack, run around the corner and inject my self.
GM: as you run a round the corner you slip as you naturally grab for the corner with your left hand. While laying on the ground, looking up, you notice that your left arm has 2 compound fractures.
Do you still want to inject the stimpack?
Player: H*$#^, yes.
GM: You feel much better after an instant. But as you attempt to stand up, your left arm gives way. The stimpack stopped the bleeding and healed the bullet hole(s ) and cuts but the compound fractures are still present.
As side note, it will take longer to heal them, since the tissue and bones will need to be recut to place them correctly for a splint.
As far-trader stated, anything instant should have some major side effects. But the side effects don't have to be delayed.
Dave Chase