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Note: I would make them more expensive (as they can get safe deep strike very reliably using daemonic icons for the Chaos Daemons Codex), but only if they can also do this with ICs attached. This would mahe them goos, but not too godd: kinda like some Land Raiders as assault transports.

If this is really a house rule, I would simply say they can assault the turn they arrive and leave the points as is.

If this is a broader discussion about what happens with the unit, I would say allowing them to assault the turn they arrive is something that should be available at a premium. 5 points per model is not unreasonable compared to the amount of damage they could do.

Give them hit and run and fleet.

Here is a potentially interesting mechanic;

If you roll a 'hit' on the scatter dice, you can assault up to the scatter distance.
Possibly force it to closest unit and/or one affected by warpflame strike.