OK...
On 20m, I worked Reunion Island at 5 W, PSK31. That's almost exactly half-way around the world (so, going further really isn't possible, short-path, anyway). Also, off the top of my head, I have worked places like Mozambique, Mauritius (near Reunion), etc., at 5W on voice. I have used lower power levels to various locations. Dipoles.
On 10m, I have worked so much QRP (mostly SSB), it's tough to recall it all. I know I have worked Paraguay at 100 mW (1/10 of a watt). Dude tells me I was an honest S9. I have also worked Europe a few times at that power level. Alaska and Hawaii, too. I am sure there are other locations. I know I have worked Australia at one watt fairly easily. At 5 W? Just about everywhere else. 5 W is really a lot of power on 10m. By the way, all of this was done on indoor dipoles.
On 6m, I have worked several countries in the Western Hemisphere, though nothing across the pond QRP. I work a LOT of 5 W QRP on 6m during really big openings, so I can monitor 2m at the same time, and not overload the receiver. 5 W is a lot on 6, and double-hop isn't that hard. I have played QRPp during big Es openings, working as low as 100 mW voice across the USA single hop (don't think I have ever worked a double at 100 mW). Fun.
I have even played QRP on 2m. Most interesting? Tropo to South Texas (nearly 700 miles) at 5 W. It was a rockin' opening. S8, I was told.
I do, BTW, have WAS/QRP as well as QRP/DXCC. QRP is just fun.
DE