The enzyme responsible is CYP1A2 if I understand correctly. Here is the paper I found that supports increased risk of a specific cardiac event to slow metabolizers. I'm not the most adept at reading medical papers so I can't verify if it claims the risk is doubled, but I think it's possible.
Coffee, CYP1A2 Genotype, and Risk of Myocardial Infarction
Marilyn C. Cornelis, BSc; Ahmed El-Sohemy, PhD; Edmond K. Kabagambe, PhD; et al Hannia Campos, PhD
JAMA. 2006;295(10):1135-1141. doi:10.1001/jama.295.10.1135
https://jamanetwork.com/journals/jama/fullarticle/202502
Coffee, CYP1A2 Genotype, and Risk of Myocardial Infarction Marilyn C. Cornelis, BSc; Ahmed El-Sohemy, PhD; Edmond K. Kabagambe, PhD; et al Hannia Campos, PhD JAMA. 2006;295(10):1135-1141. doi:10.1001/jama.295.10.1135