Can I use a 7803 (left obviously) brifter with a 6603 Front derailleur. I heard some rumbling that the actuation ratio of the new ultegra FD had changed, but am unable to find any confirmation or details

To my knowledge, all Shimano parts, of the current year, should interchange.

Until October! https://www.bikeforums.net/road-cycling/398747-shimano-79000-gossip-i-e-shimano-stepping-up.html

VERY interesting read. I missed that when it first came out. I guess Shimano has to, at least temporarily, give up across the board compatibility to stay competitive with SRAM and Campy.

I wouldn't be surprised to see the new designs work their way down the Shimano food chain over the next few years until Ultegra and 105 are again compatible with Dura Ace even if they aren't compatible with their current versions.

One aside, the new Dura Ace, SRAM and Campy are still 10-speed despite all of their other changes and redesigns. It seems 10-speed is the limit for derailleur designs, at least within current hub width and chain thickness limitations. If 11-speed or more were possible I'm sure Shimano or SRAM would have introduced it to get an advantage on their competition.

There is a limit to how thin they can make the cogs and chain links before they become completely unreliable. The next move would be to move road bikes to the same 135 OLD as mountain bikes, so that they can keep the same spacing as 10 speed, but add another gear. Obviously, short chainstay designs would make the chainline a little more wonky than it is now, but I wouldn't be surprised if they are at least looking at it.

Or go even wider like the 140 mm tandems use. But, as you noted, short chainstays would make the chainline bad and heel clearance with the chainstays could be a problem too. Apparently no one has figured out a practical way to do it.