When I started doing audaxes on it, I just threw a cheap, Ultegra Compact crankset on my PX, which replaced the same cranks you've got (my favourite 7800s).
I had Ultegra for a short spell, can't beat that for value/performance. Maybe 105, but the gunmetal would look better on your bike IMO, and there's a lot of secondhand if you're hunting for bargains
Won't get any better than the Dura Ace. I'd hunt an Ultegra 6800 and start upgrading bit by bit