First, let me say to Uber, you missed a huge problem I figured you would be the only person to point out, and aggressively.
Second, this whole thing, a cunning idea indeed, can't happen. The problem Dire, is that on your first entry missed a key word. The codex RAW says "Anytime a Swooping Hawk unit uses it's deep strike ability, it may use the grenade packs to drop a large template."
Meaning the whole unit, not singles models.
Third, the Skyleap issue. Technically, from the codex, you could move AND Skyleap. That really isn't overpowered, as you can do nothing once leaving the table, removing you from shooting and assault phases. The problem is that they become Reserves and follow Reserve rules when attempting to return on your next movement phase. So it would seem everytime you attempt to get this free jump in and Grenade Pack someone, you would be forced to roll for Reserves each turn you wanted them to deep strike back in.
It might seem overpowered that they can jump in and out, and at turn 4, return on a 2+ to do it again, but there is a big failing. You've just wasted shooting and assault phases just to reposition for a possible free template, and you have a chance of not being able to return at all. Missing out on all those lasblasters and possible sunrifle shots would prevent me from using this ability much. Pretty much only if I was cornered.
~As an edit, I thought of a powerful concept. Use the Skyleap to force big hand to hand units into wide-open close combats, or force units out that must charge closest enemies. Then when in the open, leave them hanging for the shooting gallery and Skyleap out. A beserker force would be injured greatly.~