Nakari

I did a quick test on the PTR with my hunter, using Arcane Shot with and without talent points in Mortal Shots (+30% Crit Dmg), and unequiping my helm for the tests without the meta gem:

Without Mortal Shots / without Relentless Earthstorm Diamond
Hit: 561,5 (sometimes 561, sometimes 562)
Crit: 1123 -> CritDmg Modifier = 2

Without Mortal Shots / with Relentless Earthstorm Diamond
Hit: 587,5
Crit: 1193 -> CritDmgModifier = ~2,0306

With Mortal Shots / without Relentless Earthstorm Diamond
Hit: 561,5
Crit: 1291 -> CritDmgModifier = ~2,2992

With Mortal Shots / with Relentless Earthstorm Diamond
Hit: 587,5
Crit: 1369 -> CritDmgModifier = ~2,3302

It seems the gem now only increases the critical strike damage modifier by 0,03, regardless of talents like Mortal Shots, Lethality etc... and it still works with Arcane Shot, so it's possible they just removed the effect on "real" spells.

Edit: With points in Monster Slaying (+3% damage, +3% critical strike damage) I got the following results:

With Mortal Shots / with Monster Slaying / without Relentless Earthstorm Diamond
Hit: 578
Crit: 1370 -> CritDmgModifier = ~2,3702

With Mortal Shots / with Monster Slaying / with Relentless Earthstorm Diamond
Hit: 605
Crit: 1452 -> CritDmgModifier = ~2,4

That indicates the slaying talents are still multiplicative (2,3 * 1,03 = 2,369, close enough to 2,3702), while the RES is additive.