Recent 3.2 PTR patch notes:
Shield Slam: The benefit from additional block value this ability gains is now subject to diminishing returns. Diminishing returns occur once block value exceeds 30 times the player's level and caps the maximum damage benefit from shield block value at 34.5 times the player's level.
At 80, until you get 2400 BV this has no effect, hard cap at 2760. I think this is mainly to smooth out the burst from Shield Block+Shield Slam with BV changes in 3.2, a recently reverted change had Shield Block not giving a bonus to Shield Slam anymore.. but yeah, luckily they changed it back :)