DisableCgiOverride is to prevent user from overriding cgi handler via .htaccess. What you want to do is not allowing other user (once they figure out) to use the special php handler. In shared hosting, it is not possible to dedicate that handler once it is available. Plus, there is no way to change suEXEC max connections for just one particular vhost. All vhosts have the same suEXEC max conn settings.
You can try cloudlinux(cagefs) to allocate more resource for that particular account. just make sure settings in lsws not exceed CL limit for that account.