Well, it depends on what you think of as a lackluster GPA. Are we talking about comparing two candidates, one with extensive shadowing experiences and a 3.5 versus one without and a 3.75? Or are you talking about shadowing making up for a 3.1 GPA?
Also, the breadth and long-term significance of ECs are more important than just the raw number of hours spent doing them. Whether it's 500 hours with that physician or 100 hours, I don't think makes a difference. Seeing him practice over an entire year, yes that would be important in that you've seen the ups and downs of his particular specialty over a long period of time.
If I were on the ADCOM, I'd rate each of a candidate's ECs on the following...
- What was the significance or relation to medicine for this particular EC to the candidate? Did it help them discover why they want to be a physician?
- Months/Years spent on project, not raw hours. Was this something the candidate was truly interested in showing dedication, was it just a one-time thing, or was it just done to pad the application?
- Is this something unique that would add to our class? Does the EC show that the candidate will work well with others in the class, bring a depth of experiences to the learning experience?
Like what Postbacker said, you first need to get over the minimum/cutoff hurdles of MCAT/GPA (which is generally accepted to be a 3.0 GPA and a 24 on the MCAT at a minimum, and to be really considered at all a 3.3 GPA and a 27 on the MCAT), then they start looking at your ECs.