Some performance testing and benchmarking results are here for Dell XPS 15z done with PassMark PerformanceTest ™ software (http://www.passmark.com/products/pt.htm) . Click on the image for the enlarged picture.
CPU mark comparison with other laptops
As per the test, the second generation i7 Dual core is clearly better than previous generation Quad core i7. Newer generation Quad core laptops are 75% better than new dual core i7.
Overall Passmark comparison with other laptops
Looking at the overall "PassMark Rating" result for the laptop, Dell XPS 15z i7-2620M compares well with Alienware, HP Envy, Sony VAIO laptops.