I know some local riders that can pull those speeds off quite easily. I'm not one of them.
Lots of recent time off due to work and ACL recovery has me averaging about 16 mph a ride these days.
In a flat, I'm usually on the big ring and somewhere in the middle of the cassette, plus or minus depending on a headwind or tail wind. so 53 front and 15, 17 or 19 on the rear? Something like that.
A word on cross chaining: It's absolutely fine as long as you know 100% that your bike is in tune, your chain is long enough, and your der. hanger is absolutely straight. Otherwise,
you're playing with fire on those last two biggest cogs. I've seen enough bikes with trashed rear wheels, rear derailleurs, derailleur hangers, and even frames to know it's quite easy to ruin a bike in seconds when you think "Oh, I can grab that lost cog and still ride it in the big ring." This is much more important than worrying about excess chain wear or a few watts.