I don't think medical schools care if who you are shadowing happens to be your PI as long as you get the information you need. IMO, if you have the opportunity to follow him around for a couple days just to see what it's like being a cardiologist, you might as well go for it. However, any additional experiences from a different perspective could be helpful.
Regarding LOR, I would go with just the letter from your PI. Generally, schools would rather have letters from professors that know you well (and not necessarily doctors you've shadowed). The doctors that you shadow generally do not know you long enough to write a great letter. So I think if you wanted to incorporate shadowing into a LOR, your PI may be your best bet since he/she will be able to assess you both from a research standpoint as well as your interest in medicine, leading to a very strong LOR.