Agreed, I'd make the bonus a bit lower than a Giant Owl's though, since they have leathery wings.
How about:
* When in flight, winged gnashers gain a +6 bonus on Move Silently checks.
Upon reflection, they don't need the higher Dex I proposed earlier assuming we give them the standard +2 to NA for increasing size to Large.
I'd give them a "half size" increase to their ability scores, since they ain't that much bigger than a regular gnasher.
That works out:
Gnasher, Winged
Large Magical Beast
Hit Dice: 4d10+12 (34 hp)
Initiative: +2
Speed: 30 ft. (6 squares), fly 20 ft. (clumsy)
Armor Class: 15 (-1 size, +2 Dex, +4 natural), touch 11, flat-footed 13
Base Attack/Grapple: +4/+13
Attack: Bite +8 melee (1d8+7)
Full Attack: Bite +8 melee (1d8+7)
Space/Reach: 10 ft./5 ft.
Special Attacks: Pack overrun
Special Qualities: Darkvision 60 ft., low-light vision, scent
Saves: Fort +7, Ref +6, Will +3
Abilities: Str 21, Dex 15, Con 17, Int 8, Wis 14, Cha 10
Skills: Hide +1, Jump +7, Listen +6, Move Silently +3*, Spot +6, Survival +4*
Feats: Alertness, Track
Environment: Temperate forests and hills
Organization: Solitary, pair, or pack (3-20)
Challenge Rating:
3?
Treasure: None
Alignment: Usually chaotic evil
Advancement: 5-12 HD (Large)
Level Adjustment:
+2? (cohort) or —
...
Skills: A winged gnasher has a +1 racial bonus on Listen, Move Silently, and Spot checks, and a +2 racial bonus on Hide checks.
*When in flight, winged gnashers gain a +6 bonus on Move Silently checks.
*A gnasher has a +4 racial bonus on Survival checks when tracking by scent.