The issue is not whether he is worth $8 mil, really. He's not. But the Rams have already committed the money to him; so to keep debating his inflated salary is pointless.
The issue is that the Rams would have a lot of dead cap if they cut him after this season; and only save $3 mil against the cap next year but still with a $5 mil dead cap cost.
It i unlikely to get a person who can impact a game the way Tavon does and can for $3 mil.
So the issue is: is it better to keep him at $ 8 mil and use his "gadget" abilities; or is it better to eat $5 million in dead cap, but at the same time save $ 3 mil as compared to keeping him?
In other words, it comes down to the $3 mil since the $5 mil is already a sunk cap cost that the Rams can't avoid whether they cut him or keep him.
So is he worth $3 mil? Sure.