The bonus' are definately applied for perk 5, you can test by chainsawibng with a fast click and instantly headshotting any clot and killing, whereas with another perk it doesn't always rip the head off, and even when it does it makes them stumble around for a bit. Plus the chainsaw can alt fire kill a scrake in 3 hits on suicidal at 5, dunno about primary fire but thats for lower end mobs only