I will give this Trait to the Genestealers in the game, to simulate the speed with which this sort of aliens move in films and comics...

Tyranids may gain the special trait Quicker than Light that gives the following advantages:

• While performing a half-action Move and a half-action Attack in a Round, the Genestealer may divide its movement as it wants before and after the attack (enemies may gain Free Attacks because of this).

• Genestealers engaged in combat that do not disengage may be the target of a Free Attack, but the enemy must pass a Difficult (-10) Agility Test or only be able to attack when the Genestealer has finished its move (i.e., a normal Melee Attack would not be possible).

• A creature that has Readied an action against a Genestealer must pass a Challenging (+0) Agility Test to be able to interrupt the Genestealer; otherwise, he will have to wait until the end of the action that triggered the Readied reaction to be able to carry it out.

• Jumping is a Half Action for Genestealers. Also, they can do it as part of their movement, if they still have at least half their movement allowance remaining.

• A Genestealer may use walls and ceilings of a tunnel or corridor while moving. To do this, he must pay 2 meters of movement for each meter advanced on the walls or ceiling, and he cannot advance a longer distance on a wall than previous distance on the floor or ceiling, and cannot advance a longer distance on the ceiling than what he just advanced on the wall. Also, he cannot move back from wall to ceiling if he had not previously moved on the floor.

(some examples in reply below to avoid clutter)

Example 1 : A Tyranid may dash to attack from cover by moving 6 meters, attacking a Space Marine, then moving again 6 meters to gain cover. The Space Marine needs to pass a Difficult (-10) Agility Test to gain the Free Attack.

Example 2 : A Space Marine is in the middle of a room and Readies and action to shoot at the first incoming Genestealer... A Genestealer charges at him from a nearby tunnel. The Space Marine must pass a Challenging (+0) Agility Test to interrupt the Genestealer action... If he fails the Test, he will only be able to attack after the Genestealer has charged him (and in this case maybe suffer penalties to his BS for shooting while in melee).

Example 3 : A Genestealer moves 6 meters, then attacks the Space Marine, then jumps up onto the rafters of the ceiling (see page 207 of the Core Rulebook), rolls its Challenging (+0) Agility Test and succeeds with a 45 (it has an Agility of 60). It jumps up 7,5 meters (1,5 meters + half its Strength Bonus), grabs the rafter and needs to pass a Challenging (+0) Strength Test to climb on top of it in the same move. Again, the Space Marine needs to pass a Difficult (-10) Agility Test to gain the Free Attack.

Example 4 : A Genestealer moves its Full action speed along a tunnel (24 meters) blocked by a Space Marine 10 meters from him. The Genestealer may move on the floor 4 meters, then on the wall 4 meters, then another 4 meters on the ceiling, another 4 meters on the opposite wall, finally continue for 12 more meters on the floor, having left the Space Marine behind him.

genestealers are lethal enough , this could make them unstoppable and very frustrating for the players to face

I agree with Hardrainfailling, the genestealers are hard enough. However, in my games they always have been capable of wall running...because in "Aliens", the xenomorphs do it.

So it seems so... correct. And does not give a game breaking combat advantage.

Well, I use the Living Errata that says that Rending Claws deal 1d10R damage with Pen 5. That means Genestealers deal 1d10+12R damage with Penetration 5 (or Penetration 10 if Razor Sharp activates). So usually Space Marines (TB8) will suffer 1+1d10 or 4+1d10 Wounds... Even rolling maximum damage, the Genestealer will need 3 attacks to kill a Space Marine (and that not counting Dodge or Parry)... so hard, but not overwhelming.

A Space Marine armed with a Bolter (standard weapon) deals 1d10+9X, Pen 4, Tearing. That means he'll deal 1+1d10 Tearing damage to the Genestealer, so he has more or less the same capacity to kill a Genestealer in 3 attacks, but that does not count that the Space Marines always have some extra tricks... Squad Support, Talents that give him extra damage, easier Fury because of rolling two dice for damage and automatic confirmation of Fury, etc.

This means that an interesting combat would be having the same number of Tyranids than players, and a tough one would be adding 50% more to the number. The new trait is there for adding some extra tactical options and simulating the way Genestealers move in movies, games, etc. So I think the real danger with Genestealers is having too many in a combat and thus overwhelming the players. That-s the reason I never place more Genestealers than 1,5 times the number of Space Marines.

I'd remember at the moment that you are also using the Final Sanction genestealers which are rather tame compared to the purestrain version within Mark of the Xenos.
Lightning attack, Multiple arms and Step Aside now come as standard and even with the errata'd stats they can mince your marines quickly. 8-)

I sprang these on my players as we hit the spaceport bit within Oblivion's Edge. They passed the awareness check to spot that there were significant differences betwean these 'stealers and the ones they had previously killed but they were still shocked when these things charged straight across the killing grounds and tore through the remaining pdf while sidestepping significant amounts of firepower sent back at them.