You would need someone with access to CloudFlare POPs to run a traceroute from say, their Sydney POP to a GCE us-central instance to be able to tell whether this is really of much value apart from reducing costs at specific IX/peering points (which is actually of significant value, but I don't think this improves performance as much as you would expect -- the Interconnect is for reducing bandwidth costs for Google).
https://www.cloudflare.com/google/