When DJ mash-ups appeared a few years back, they were all about linking strange bedfellows — Nirvana and Destiny’s Child, the Strokes and Christina. The best ones made you hear something new in both songs involved — which Collision Course does only in snatches. Billed as the first commercially released mash-up record, this six-song EP is well-constructed: The mix of brassy rhymes and heavy electro-rock on “Big Pimpin’/Papercut” and “Dirt Off Your Shoulder/Lying From You” — featuring new verses and music from the band and Jay — is way more interesting than most rap metal. But too often Jay’s world-class flow leaves Linkin Park in the dust, and the juxtapositions are pure novelties and less fun than they should be. If Jay really needed another batch of out-there beats, he only had to hit up Timbaland again.