I think it depends on what content is the web-server serving. But if you have properly configured web-services, then your network-capacity and/or disk i/o is bottleneck, not cpu...
Next thing, your cpu has actually only 4 cores, the other 4 are fakes. Because I do not know any web-server which could use hyperthreading effectively, you are actually over-commiting your cpu resources, even if there is no other VM on your esxi-server. Check this report. It is a nice example how sometimes with "less" you can achieve "more"...
