Weirdly, I don't think it really mattered in the end. Whether he had it installed, even if he just put it in a "shallow" cut, no matter what happened he gets what he wants.
Even if the heart was still in the case Himegama, his femme fatale secretary, would stop anyone who tried to open it to confirm.
On top of that Yoshi had proven to be nothing but a manipulative bastard so if he's left alive he can do more harm, but him dying seems to be part of his plan so really he wins either way. Mika even seems to realize this as he pulls the trigger but they're both too far gone to stop it.