Why Tahm Kench damage are buffed?
Tongue Lash (Q) damage increased to 80/130/180/230/280 from 80/125/170/215/280 Abyssal Voyage (R) range increased to 4500/5500/6500 from 4000/5000/6000 http://www.surrenderat20.net/2016/06/617-pbe-update.html
HE IS literally the support who deals the biggest amount of damage

All of these I have seen as support, and ALL of them do more damage(and its more sustained too) than Tahm. Zyra alone can out damage a fed carry, as long as they are not 6-10+ kills ahead of her.