On client's servers I have that hosts one big website i always run lsws as apache or nobody. I dont think it makes a difference though as far as performance. And to be honest I have seen the phpsuexec on latest lsws versions to use only a fraction more resources as usual.
Here is some reasons you may wanna run a one site server as nobody or apache
IN case the site is exploited and php shell is got on the site, they would only have read/write access to directories and files which are world readable and writable. With common chmod they would be able to read but not write to files and dirs unless they are world writable.
So if you dont have any world writable directories then they would not be able to delete your site or overwrite files.
With suphp if they have shell they now own the files and can write, modify, and delete at will but the benefits on shared hosting with this is that they most likely will not be able to affect other users and you can keep secure permissions on files and directories.
So if you do not have any other users on your server then there should be no problem running as apache/nobody. As far as speed increase or performance increase I think it would be the same.
And thats my logic on selecting suexec or nobody/apache. I think its just common sense, although I have brought up this subject before at webhostingtalk and turned into a huge argument so I think its a thing of preference.