The base BV for all Clan units is calculated as if they have 4 gunnery, 5 piloting. Players may then choose to upgrade their pilots further, but have to pay the same BV modifiers as with units of any tech base.
No unit comes with skills that are "fixed" at any level; for example, if playing a simple 6000 BV game, a player may choose to field a Timber Wolf with 4/5 skills---they are not forced to use it with a 3/4 pilot.
Hope that helps.
