As I've found on the net, webgarden only gives you more headace and not really adds any benefits to your site performance if you don't know what to manage it properly.
For normal sites, webgarden could add xN consumption of memory + cpu
http://dotnetslackers.com/articles/iis/Why-You-Shouldnt-Use-Web-Gardens-in-IIS-Week-24.aspx
http://serverfault.com/questions/81689/web-gardens-are-they-good-or-bad-or-what
Web gardens was designed for one single reason – Offering applications that are not CPU-bound but execute long running requests the ability to scale and not use up all threads available in the worker process. - IIS team
Rather, we should focus on Web farms for large sites that require multiple hosting servers to balance the network.
