There is as much overhead downloading the file from the data center as there is from a remote location. There's no such thing as a "file ready and queued" unless you bring WAN accelerators into play, which will cost you upwards of $50,000 (which would be a steal - decent solutions are six figures and up). Just because the data center has a 100Mbps uplink doesn't mean it's fetching the file for you at that speed if it's just a routing hop in the middle every goes through - it's just a conduit for the 20Mbps limited pipe at one end. And it certainly doesn't make any sense to cache files that large that you're very unlikely to ever serve again.
What is this proposed "server" doing at the data center? If it's just a router with RouterOS on it it doesn't "queue files" for clients. It's just a router.
On a side note, if I may suggest it, you may want to be more careful with units and capitalization. There's a difference between Mbps, MB/s, and MB - and most of it is made non-ambiguous by capitalization. I found your example very hard to read due to this.
Specific answers require specific questions. When in doubt, post the output of "/ip address print detail", "/ip route print detail", "/interface print detail", "/ip firewall export", and an accurate network diagram.