Tamil Sri Lankans share the common genetic heritage as Southern Tamil Indians.I refer to the quote below to provide further information on this topic.
"India, being a center stage for modern human dispersals, is a diverse human genetic pool drawn from widely divergent cultures, religions, languages, and geographic backgrounds (Gadgil et al. 1997). Human diversity in India is characterized by 4,635 documented communities, constituting 2,205 major population groups (Singh 1998), 461 tribal communities (Singh 1992), 20 major languages, and approximately 750 dialects (Kosambi 1991). These groups reflect diverse ethnic populations (Australian, Indo-Caucasian, Indo-Asian, and African descent) and linguistic groups (Indo-European, Dravidian, Austro-Asiatic, and Tibeto-Burman). Most of these ethnic populations belong to the Hindu religious fold, which is socially systematized into endogamous hierarchical castes and subcastes (Mukherjee et al. 2001).(Census of India 2001).
Genetic Affinity Between Diverse Ethnoreligious Communities of Tamil Nadu, India:
Essentially there is European African & Asian genetic influence within the Indian population. Hence the terms Indo-Caucasian, Indo-African, Indo-Asian. I don’t think the generic term of Indian is suffice when referring to Indians. Within the group large clear separate racial groups exist with varied origins.
Getting back to the topic of athletics and winning medals the main limiting factor is cultural and socio-economic. The gene pool is phenomenal to so say the least.