SEO advantages of dedicated IP and dedicated server have been debated for long time. One must understand that a dedicated server for every domain is a costly proposition. It is a well established fact that in order to get better ranking on search engines, one needs to focus on off-site SEO rather than being overly concerned with type of hosting and IP address.
Site speed is a crucial factor for enhanced browsing experience and greater visit frequency. Site speed can be augmented with help of other ways that are much cheaper. This brings us to the point of
dedicated IP address for better SEO. One should remember that an IP address is as good as your cell number and it does not matter if you change your cell number.
Although a possibility of your site being flagged due to illegitimate activities on other account that share the same IP address, it is a rare phenomenon and one need not get obsessed with it. You can always monitor your co-hosted accounts with help of tools such as Google webmaster, or reverse IP checker. Hence it can be safely concluded that you need not have a
dedicated server and a dedicated IP address exclusively for better SEO. Instead you can focus on other avenues to reduce page loading speeds